I dont know if its already posted here, but try this:

1) Plug the charger to wall and to n900.
2) blame if doesnt work
3) reboot your phone without unplugging anything. I use "sudo gainroot" and "reboot" from xterm, but do is as you want
4) be happy. When the phone reboots, it starts charging.

Not every phone works with this method, but a LOT do.