Seeing as Linux has yet to publish the GPL code for this Firmware, and I don't know how to check it through a console command (uname isn't recognized), I can't say which kernel it is, but I'd like to know.
sample from beginning of dmesg for wrt54g (with 2.4-brcm):
root@officegw:~# dmesg CPU revision is: 00029008 Primary instruction cache 16kB, physically tagged, 2-way, linesize 16 bytes. Primary data cache 8kB, 2-way, linesize 16 bytes. Linux version 184.108.40.206 (nbd@baustelle) (gcc version 3.4.6 (OpenWrt-2.0)) #27 Wed Jan 21 02:43:07 CET 2009 Setting the PFC to its default value Determined physical RAM map: memory: 01000000 @ 00000000 (usable) On node 0 totalpages: 4096 zone(0): 4096 pages. zone(1): 0 pages. zone(2): 0 pages. Kernel command line: root=/dev/mtdblock2 rootfstype=squashfs,jffs2 init=/etc/preinit noinitrd console=ttyS0,115200 CPU: BCM5352 rev 0 at 200 MHz Using 100.000 MHz high precision timer. Calibrating delay loop... 199.47 BogoMIPS Memory: 14260k/16384k available (1425k kernel code, 2124k reserved, 100k data, 84k init, 0k highmem)