I assume you don't have U-boot installed? (in which case you could still recover your system booting off an sd card).

If /usr/bin is not there, Maemo is not there anymore (in the bootable sense of the word). Meaning your only option is to reflash the rootfs. Meaning you need a working USB port.

You said you've tried flashing following this thread (http://talk.maemo.org/showthread.php?t=86547), which I personally find a bit to messy as well as confusing (flashing only kernel will not help *you*, you don't need a new kernel, you need a new / partition).

First things first: which OS are you using? 64-bit Windows is a notorious no-go (although it may work after all). The best recommended OS is a 32-bit Linux.

Try to get a live CD from a suitable distro (Ubuntu should be fine) and then follow the instructions here: http://wiki.maemo.org/Updating_the_tablet_firmware

The above Wiki may be a bit confusing, as it deals with various cases as well as other Maemo devices.

In essence, you need to:
1. remove battery of N900 and wait a minute or two (just for the fun of it)
2. run the following command on the PC, as root:
# flasher-3.5 -F combined.bin -f -R
(where combined.bin is your combined/fiasco firmware image)
3. when it says "suitable usb device not found, waiting,
slide out keyboard, hit "u" and keep it pressed. put battery in:
if it turns on automatically it should start flashing automatically,
if not, use your third hand to turn the N900 on. the flashing should start.

If the flasher doesn't recognize the N900 or the N900 doesn't recognize the flasher, then it will continue booting until it stops/reboots. Try again a couple of times (always removing the battery, etc.).

Try also connecting the USB to different ports of your PC (some ports may be hubbed.)

If nothing works you need to have your USB port repaired.