Sipdroid, recently added video and probably can communicate with the N900. It also integrates with the dialer (AFAIK):
Here's the project page with video: http://sipdroid.org/

I've noticed that video calling has been weak at best, largely due to a lack of standardization. Most players seem to offer proprietary solutions in an effort to lock users into their system each with a unique IM, protocol and codec(s). These islands are small, segmented, and annoying.

I'm hoping that Google can remedy this by pushing open standards (eg. SIP/webm) and inviting others to join in, effectively consolidating all would-be chatters in a decentralized system.