I have a Tekkeon MP3450, and it has trouble charging the N900. Intofx's suggestion of unplugging and replugging will sometimes work -- it seems to depend on how fast I plug it back in and gravity, perhaps? It is very fiddly and takes many, many tries to get it to catch, but once it starts charging I can move it around and do what I like.

I hope there is a software fix/hack for this someday, though.

*edit* Okay, nevermind the confusion -- it is consistently plugging it in to the N900 slowly that makes it work. It takes a certain speed to do, but after a bit of practice I can get it to take every time.
WEIRD, though.