This may not be the case for you but there are internal conditions on corrupted N900's where flashing mode never presents itself even if the USB port is working fine.
Some will tell you this is not the case and there are plenty things to do right if you read the wiki, but I had a so called new $200 Hong Kong N900 (new case old junk board) that was wonky and I was only able to get flashing mode on a reboot, it faild a flash at 43% and I was never able to flash it again, bricked.
Follow the instructions on the wiki and post command line output from
sudo tail -f /var/log/messages
or
sudo tail -f /var/log/syslog

If you want to see a fumbling attempt peruse this thread http://talk.maemo.org/showthread.php?p=1210183

I start the flasher, plug the computer and N900 in with battery out, and hold the U key as I insert the battery, that usualy works when nothing else does.

If possible use and alternative method to charge your dead battery, this will also prevent flashing. If you can buy a external charger or use another phone. Worst case(high risk of burnout) I have made an emergency external charger from an old USB cable and a cut paper clip, it was risky but I got enough power in the battery to boot the phone for charging. I held the battery in my hand to feel heat and only gave it five minutes.