How can I stop the bootloader unlock process? My 4 year old turned on my phone (Nexus S, 2.3.4 I think) while pressing the volume button. Apparently, this triggers the bootloader unlock process, which I don't want.

The phone is now stuck on the "unlock bootloader" screen and I'm afraid to lose lots of unbacked-up stuff. The soft keys have disappeared, and all I see is the unlock icon and no way to back out of this process.  Any ideas?

Do I let the battery die and hope it offers me the password page next time? Do I connect to a computer and do something or yank the battery?
Your phone is basically waiting for input from you, not really doing anything under the hood, on the "Unlock Bootloader" screen. It should be safe to simply pull the battery and then reboot the phone normally (one corroborating source: Google's help forums).