This is under discussion in the "Video Calls with Skype N900" thread. From what's been written there, it appears the issue is that Skype on Maemo 5 exclusively uses the H.264 codec for video, and few other versions of Skype support H.264. Skype on the iPhone does not, and only very recent desktop versions of Skype do.

Your only hope is that Skype releases a new revision of Skype for iPhone that supports H.264. The iPhone already uses H.264 for Facetime, so it's not a hardware limitation on the iPhone side. And an updated, more capable version of Skype for iPhone is infinitely more likely than any new version of Skype for Maemo 5.