There were a couple of chinese devices that used maemo. But I believe some of the maemo 5 UI is a closed source module, so i don't know if it can be used by others.

Nokia's biggest problem is that it is moving way to slowly in the high-end smartphone segment. The N900 is a lovely hackable geek toy, that is really innovative in a lot of ways. But is is also very much "out of date" to be a high-end smartphone.
High-end smartphone users currently expect a capacitive touch screen with a fully touch based UI, a slim design, and a fast an vibrant application store. The n900 doesn't have these things or they are very much in beta.

Android is evolving at a really fast pace. the latest 2.3 now also supports SIP calling as a integrated service if I'm not mistaking (something that was unique to Maemo 5 up to this point).