Hi,

  The following has been used to connect E1820 into OpenWrt- backfire 10.03, with successful
  (top dmesg output)
---------------------------------------------------------------------------------------------------------------------------
Linux version 2.6.32.10 (openwrt@wrt1.marcant.net) (gcc version 4.3.3 (GCC) ) #20 Tue Apr 6 15:53:48 CEST 2010
CPU revision is: 00029007 (Broadcom BCM3302)
ssb: Core 0 found: ChipCommon (cc 0x800, rev 0x09, vendor 0x4243)
ssb: Core 1 found: IEEE 802.11 (cc 0x812, rev 0x07, vendor 0x4243)
ssb: Core 2 found: Fast Ethernet (cc 0x806, rev 0x07, vendor 0x4243)
ssb: Core 3 found: USB 1.1 Device (cc 0x818, rev 0x01, vendor 0x4243)
ssb: Core 4 found: USB 1.1 Host (cc 0x817, rev 0x01, vendor 0x4243)
ssb: Core 5 found: MIPS 3302 (cc 0x816, rev 0x01, vendor 0x4243)
ssb: Core 6 found: MEMC SDRAM (cc 0x80F, rev 0x01, vendor 0x4243)
ssb: Core 7 found: PCI (cc 0x804, rev 0x0A, vendor 0x4243)
---------------------------------------------------------------------------------------------------------------------------

root@openwrt> usb_modeswitch -e
--------------------------------------------------------------

* usb-modeswitch: handle USB devices with multiple modes
* Version 1.1.2alpha (C) Josua Dietze 2010
* Based on libusb0 (0.1.12 and above)

! PLEASE REPORT NEW CONFIGURATIONS !

---------------------------------------------------------------------------------------------------------------------------

  /etc/usb_modeswitch.setup
----------------------------------------------------------------------------------
########################################################
# Huawei E270+  (HSPA+ modem)
# Huawei E1762
# Huawei E1820
#
# Contributor: Paranoid Paranoia

DefaultVendor=  0x12d1
DefaultProduct= 0x1446

TargetVendor=   0x12d1
TargetProduct=  0x14ac

MessageContent="55534243123456780000000000000011060000000000000000000000000000"

/etc/config/network
----------------------------------------------------------------------------------
config 'interface' 'ppp0'
        option 'device' '/dev/ttyUSB0'
        option 'proto' '3g'
        option 'apn' 'INTERNET'
        option 'service' 'hsdpa'
        option 'mtu' '1492'

/etc/modules.d/60-usb-serial
----------------------------------------------------------------------------------
usbserial vendor=0x12d1 product=0x14ac

/etc/rc.d/S95done
----------------------------------------------------------------------------------
#!/bin/sh /etc/rc.common
# Copyright (C) 2006 OpenWrt.org

START=95
boot() {
        [ -d /tmp/root ] && {
                lock /tmp/.switch2jffs
                firstboot switch2jffs
                lock -u /tmp/.switch2jffs
        }

        # process user commands
        [ -f /etc/rc.local ] && {
                sh /etc/rc.local
        }

        # set leds to normal state
        . /etc/diag.sh
        set_state done
        usb_modeswitch
}

root@openwrt> dmesg
--------------------------------------------------------------
usbserial_generic 1-1:1.0: generic converter detected
usb 1-1: generic converter now attached to ttyUSB0
usbserial_generic 1-1:1.1: generic converter detected
usb 1-1: generic converter now attached to ttyUSB1
usbserial_generic 1-1:1.2: generic converter detected
usb 1-1: generic converter now attached to ttyUSB2
usbserial_generic 1-1:1.3: generic converter detected
usb 1-1: generic converter now attached to ttyUSB3
usbserial_generic 1-1:1.4: generic converter detected
usb 1-1: generic converter now attached to ttyUSB4
usbserial_generic 1-1:1.5: generic converter detected
usb 1-1: generic converter now attached to ttyUSB5
usbcore: registered new interface driver usbserial_generic

root@openwrt> ifup ppp0
--------------------------------------------------------------
SIM ready
PIN set successfully

root@openwrt> ifconfig
--------------------------------------------------------------
ppp0      Link encap:Point-to-Point Protocol
          inet addr:188.140.46.204  P-t-P:10.64.64.64  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1492  Metric:1
          RX packets:47 errors:0 dropped:0 overruns:0 frame:0
          TX packets:55 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:3507 (3.4 KiB)  TX bytes:4093 (3.9 KiB)

root@openwrt> ping www.google.com
--------------------------------------------------------------
PING www.google.com (209.85.229.104): 56 data bytes
64 bytes from 209.85.229.104: seq=0 ttl=54 time=170.926 ms
64 bytes from 209.85.229.104: seq=1 ttl=54 time=189.186 ms
64 bytes from 209.85.229.104: seq=2 ttl=54 time=131.618 ms
64 bytes from 209.85.229.104: seq=3 ttl=54 time=130.702 ms
64 bytes from 209.85.229.104: seq=4 ttl=54 time=130.068 ms
64 bytes from 209.85.229.104: seq=5 ttl=54 time=128.887 ms
C64 bytes from 209.85.229.104: seq=6 ttl=54 time=127.449 ms
^C
--- www.google.com ping statistics ---
7 packets transmitted, 7 packets received, 0% packet loss
round-trip min/avg/max = 127.449/144.119/189.186 ms

mv /etc/usb_modeswitch.setup /etc/usb-modeswitch.conf

Install the usb-modeswitch and the usb-modeswitch-data packages.

I'll write my experience with Huawei E1762 and OpenWrt 10.03.1-rc4 here, even though the title says E1820 (this thread seems to be currently the top result in Google for terms "openwrt e1762").

I struggled quite long time following different guides, like this thread and http://josefsson.org/openwrt/dongle.html

Finally the breakthrough for me was to install kmod-usb-serial-option and leave /etc/modules.d/60-usb-serial untouched (ie. don't put any vendor or product code there).
Also I didn't need to create any /etc/usb-modeswitch.conf file. I installed the usb-modeswitch and usb-modeswitch-data packages, though. I also installed a lot of usb related modules during the trial-and-error session, many of those maybe unnecessary, and comgt.

dmesg:
USB Serial support registered for GSM modem (1-port)
option 1-1:1.0: GSM modem (1-port) converter detected
usb 1-1: GSM modem (1-port) converter now attached to ttyUSB0
option 1-1:1.1: GSM modem (1-port) converter detected
usb 1-1: GSM modem (1-port) converter now attached to ttyUSB1
usbcore: registered new interface driver option
option: v0.7.2:USB Driver for GSM modems

(Last edited by Akhwel on 3 Feb 2011, 12:04)

OpenWRT r26276 on TL-MR3420 (v1) with a Huawei E1820.

opkg install comgt kmod-usb-serial-option usb-modeswitch  usb-modeswitch-data
vi /etc/config/network
etc/init.d/network restart