Then again, having looked at the pictures in this post which documents with photographs how to replace the N8 battery I'm really not sure how Powermat intend to support the N8 unless the Powerpak battery is the entire inductive charging system with no replacement back cover required (in which case an X6/5800 Powerpak should work with the N900, unless there are other technical issues outstanding).

It certainly doesn't look like you can easily replace the back cover on the N8 (note that in the article they switch between a blue and black N8 in the photographs).