It's partially fixed. It works when the disk is plugged after booting. But there ist still no /dev/sda* created when the disk is already plugged in before powerup.
root@OpenWrt:/# ls /dev/scsi/host0/bus0/target0/lun0 -l
brw------- 1 root root 8, 0 Jan 1 1970 disc
brw------- 1 root root 8, 1 Jan 1 1970 part1
brw------- 1 root root 8, 2 Jan 1 1970 part2
root@OpenWrt:/# ls /dev -l
crw------- 1 root root 5, 1 Jan 1 00:00 console
drwxr-xr-x 1 root root 0 Jan 1 1970 discs
crw-rw-rw- 1 root root 1, 7 Jan 1 1970 full
crw-r----- 1 root root 1, 2 Jan 1 1970 kmem
crw-r--r-- 1 root root 1, 11 Jan 1 1970 kmsg
srw-rw-rw- 1 root root 0 Jan 1 00:00 log
crw-r----- 1 root root 1, 1 Jan 1 1970 mem
drwxr-xr-x 1 root root 0 Jan 1 1970 mtd
drwxr-xr-x 1 root root 0 Jan 1 1970 mtdblock
crw-rw-rw- 1 root root 1, 3 Jan 1 1970 null
crw-r----- 1 root root 1, 4 Jan 1 1970 port
crw-rw-rw- 1 root root 5, 2 Jan 1 1970 ptmx
drwxr-xr-x 2 root root 0 Jan 1 00:00 pts
crw-r--r-- 1 root root 1, 8 Jan 1 1970 random
lr-xr-xr-x 1 root root 10 Jan 1 00:00 root -> mtdblock/2
drwxr-xr-x 1 root root 0 Jan 1 1970 scsi
drwxr-xr-x 1 root root 0 Jan 1 1970 shm
drwxr-xr-x 1 root root 0 Jan 1 1970 tts
crw-rw-rw- 1 root root 5, 0 Jan 1 1970 tty
crw-r--r-- 1 root root 1, 9 Jan 1 1970 urandom
crw-rw-rw- 1 root root 1, 5 Jan 1 1970 zero
Jan 1 00:00:16 (none) user.notice kernel: Creating 5 MTD partitions on "Physically mapped flash":
Jan 1 00:00:16 (none) user.notice kernel: 0x00000000-0x000a0000 : "cfe"
Jan 1 00:00:16 (none) user.notice kernel: 0x000a0000-0x007e0000 : "linux"
Jan 1 00:00:16 (none) user.notice kernel: 0x00164400-0x00260000 : "rootfs"
Jan 1 00:00:16 (none) user.warn kernel: mtd: partition "rootfs" doesn't start on an erase block boundary -- force read-only
Jan 1 00:00:16 (none) user.notice kernel: 0x007e0000-0x00800000 : "nvram"
Jan 1 00:00:16 (none) user.notice kernel: 0x00260000-0x007e0000 : "OpenWrt"
Jan 1 00:00:16 (none) user.warn kernel: PCI: Enabling device 0000:01:02.2 (0000 -> 0002)
Jan 1 00:00:16 (none) user.warn kernel: PCI: Fixing up device 0000:01:02.2
Jan 1 00:00:16 (none) user.info kernel: ehci_hcd 0000:01:02.2: EHCI Host Controller
Jan 1 00:00:16 (none) user.info kernel: ehci_hcd 0000:01:02.2: new USB bus registered, assigned bus number 1
Jan 1 00:00:16 (none) user.info kernel: ehci_hcd 0000:01:02.2: irq 2, io mem 0x40000000
Jan 1 00:00:16 (none) user.info kernel: ehci_hcd 0000:01:02.2: USB 2.0 started, EHCI 1.00, driver 10 Dec 2004
Jan 1 00:00:16 (none) user.info kernel: usb usb1: configuration #1 chosen from 1 choice
Jan 1 00:00:16 (none) user.info kernel: hub 1-0:1.0: USB hub found
Jan 1 00:00:16 (none) user.info kernel: hub 1-0:1.0: 2 ports detected
Jan 1 00:00:16 (none) user.info kernel: Initializing USB Mass Storage driver...
Jan 1 00:00:16 (none) user.info kernel: usb 1-1: new high speed USB device using ehci_hcd and address 2
Jan 1 00:00:16 (none) user.info kernel: usb 1-1: configuration #1 chosen from 1 choice
Jan 1 00:00:16 (none) user.info kernel: scsi0 : SCSI emulation for USB Mass Storage devices
Jan 1 00:00:16 (none) user.info kernel: usbcore: registered new driver usb-storage
Jan 1 00:00:16 (none) user.info kernel: USB Mass Storage support registered.
Jan 1 00:00:16 (none) user.warn kernel: es per conntrack
Jan 1 00:00:16 (none) user.debug kernel: usb-storage: device found at 2
Jan 1 00:00:16 (none) user.debug kernel: usb-storage: waiting for device to settle before scanning
Jan 1 00:00:16 (none) user.warn kernel: ip_tables: (C) 2000-2006 Netfilter Core Team
Jan 1 00:00:16 (none) user.info kernel: TCP vegas registered
Jan 1 00:00:16 (none) user.info kernel: NET: Registered protocol family 1
Jan 1 00:00:16 (none) user.info kernel: NET: Registered protocol family 17
Jan 1 00:00:16 (none) user.info kernel: 802.1Q VLAN Support v1.8 Ben Greear <greearb@candelatech.com>
Jan 1 00:00:16 (none) user.info kernel: All bugs added by David S. Miller <davem@redhat.com>
Jan 1 00:00:16 (none) user.warn kernel: VFS: Mounted root (squashfs filesystem) readonly.
Jan 1 00:00:16 (none) user.info kernel: Mounted devfs on /dev
Jan 1 00:00:16 (none) user.info kernel: Freeing unused kernel memory: 132k freed
Jan 1 00:00:16 (none) user.warn kernel: Algorithmics/MIPS FPU Emulator v1.5
Jan 1 00:00:16 (none) user.notice kernel: Vendor: TinyDisk Model: 2006-06-22 Rev: 0.00
Jan 1 00:00:16 (none) user.notice kernel: Type: Direct-Access ANSI SCSI revision: 02
Jan 1 00:00:16 (none) user.notice kernel: SCSI device sda: 4096000 512-byte hdwr sectors (2097 MB)
Jan 1 00:00:16 (none) user.notice kernel: sda: Write Protect is off
Jan 1 00:00:16 (none) user.debug kernel: sda: Mode Sense: 00 00 00 00
Jan 1 00:00:16 (none) user.err kernel: sda: assuming drive cache: write through
Jan 1 00:00:16 (none) user.notice kernel: SCSI device sda: 4096000 512-byte hdwr sectors (2097 MB)
Jan 1 00:00:16 (none) user.notice kernel: sda: Write Protect is off
Jan 1 00:00:16 (none) user.debug kernel: sda: Mode Sense: 00 00 00 00
Jan 1 00:00:16 (none) user.err kernel: sda: assuming drive cache: write through
Jan 1 00:00:16 (none) user.info kernel: /dev/scsi/host0/bus0/target0/lun0: p1 p2
Jan 1 00:00:16 (none) user.notice kernel: sd 0:0:0:0: Attached scsi removable disk sda
Jan 1 00:00:16 (none) user.debug kernel: usb-storage: device scan complete
Jan 1 00:00:17 (none) user.info kernel: b44: eth0: Link is up at 100 Mbps, full duplex.
Jan 1 00:00:17 (none) user.info kernel: b44: eth0: Flow control is off for TX and off for RX.
Jan 1 00:00:17 (none) user.info : sed: /etc/modules: No such file or directory
Jan 1 00:00:18 (none) user.notice root: waiting for sda2...........mount: Mounting /dev/sda2 on /tmp/sda2 failed: No such file or direcy
Jan 1 00:00:18 (none) user.notice root: waiting for sda1...........mount: Mounting /dev/sda1 on /tmp/sda1 failed: No such file or direcy
Jan 1 00:00:18 (none) user.notice root: waiting for sda...........
Jan 1 00:00:18 (none) user.info kernel: wlan: 0.8.4.2 (0.9.0)