As far as I know these kind of deals are partnerships. Hence, if Nokia wants more devices sold then killer software on them is a must (skype video or whatever your idea is..) while at the same time this provides Skype with some more potential users (if already a user then they can retain you). However, if your only going to sale 10,000 units then it won't be worth it to Skype to spend their energy on your device. Or who knows maybe Skype sales a license to Nokia for each unit.

In sum, you need two sides to make a deal.