Aha - turning the phone on with the keyboard open displays "Loading BOOT MENU..." on the Nokia logo screen, then shows the three boot-choices - is this a standard n900 feature? (Incidentally, I haven't seen the large 'Backup Menu' menu, so it probably isn't installed).

Here are the three choices:

Maemo 5 (Internal Nand)
Recovery (Internal flash)
Power Off (only when not charging)

and, at the bottom of the page:

Press HOME/CTRL key for USB mass storage mode
Press HOME/SHIFT key for USB network mode

(What are these keys on the n900? And, is there any way to connect to this crippled phone wirelessly?)

When I select the first boot method, I get the endless Five-Dot back-and-forth.

When I try the second method, "Booting from nand (mtdblock4 jffs2)..." and "Starting recovery.sh..." is displayed for about one to twenty seconds on a bright screen, then the screen goes dark, and at some point the phone shuts off (only the on button works). I recall having a short (± 20-line) script named 'recovery.sh' in the root directory.
Note: this is similar to what happened originally - after the phone went dark for a while, I paniced and yanked the battery. If, however, the phone had shut off, then removing the battery shouldn't have harmed anything (I'm guessing the phone hadn't shut off, since that would seem a pointless mode).

Are the last two instructions of any help?
Are there more special booting methods?