If your battery is low (not flat, but low) it won't flash any how..
Normal low battery behaviour I've seen is to power off after trying to boot, not to reboot after trying to boot (ie reboot continuously)

Just because the charging light is flashing doesn't mean your battery is charged.. A good check is to try and press the U button whilst powering on.. if the screen stays dark, displays a Nokia logo and a little USB icon in the top right of the screen, it is fairly safe to say the battery is mostly full.. certainly full enough to flash and recover your phone..

if, however, the display lights up and the phone tries and fails to boot, you're going to need to find an alternative way to charge your battery.. someone else with an N900, X6, 5800 or 5230 would do since they all use a BL-5J battery..

Once you've got a charged battery, your device might wake up of its own accord.. failing that, it's probably time for a visit from mr flasher-3.5 to kick things into action