If all else fails, use the power switch 4 times, or a script to boot back to the other partition:
#!/bin/sh
#hacked from /lib/upgrade/linksys.sh
cur_boot_part=`/usr/sbin/fw_printenv -n boot_part`
target_firmware=""
if [ "$cur_boot_part" = "1" ]
then
target_firmware="kernel2"
fw_setenv boot_part 2
fw_setenv bootcmd "run altnandboot"
elif [ "$cur_boot_part" = "2" ]
then
target_firmware="kernel1"
fw_setenv boot_part 1
fw_setenv bootcmd "run nandboot"
fi
# re-enable recovery so we get back if the new firmware is broken
fw_setenv auto_recovery yes
echo "$target_firmware"
reboot