I ran into an error much like this recently (Pro Tip: don't put keychain in .bashrc or .bash_profile like the desktop install guides say; you will receive much pain and suffering), and was able to fix it using the MeeGo rescue initrd tools: http://wiki.meego.com/ARM/N900/Using_Rescue_Initrd

The "recovery terminal" mentioned there is largely valueless (the keymap's incorrect, so you can't type anything that requires the blue arrow shift key, which includes such vital characters as / and -), but that setup + USB Storage toggle + a Rescue Is Possible bootdisk I keep with my laptop allowed me to get to the broken .bashrc that was keeping me from booting.

(There's newer versions of that initrd image and kernel floating around in those repositories, but I haven't tried them to see if the keymap is fixed in those.)

Alas, I only discovered this after I'd tried reflashing the rootfs, so I still have some recovery to do. But at least I didn't have to wipe my eMMC...