My understanding is that that IM+ for Skype uses BOTH data and cell minutes. Please correct me if I'm wrong but from reading their FAQ on their site (SHAPE Services - IM+ for Skype Software FAQ
IM+ uses data connection to connect to the skype network, send messages, keep your list in sync, and you "online".
When making or receiving a call... the "callback number" you setup when installing and running Skype is used. That is, Skype actually calls your cell phone to establish the call - you are now paying for (A) redirection to your phone (B) Skype minutes to the caller you're connected to - this might be free if you have the unlimited skype plan within the US and Canada and are talking to someone in those areas; or you're paying the skype international rates AANNND (C) Any airtime minutes charged by your cell phone service provider to establish that incoming call.
Really I can't see the IM+ software being that much of a benefit unless you call overseas often... if you do then the skype rates definitely beat out the 1.50/min or whatever international rate your cell provider might charge.
But for me, with most my calls in the US, and its using plan minutes anyways to establish the link-back call... I might as well call the person directly on my phone.
My $.02 - I think I've decrypted how the application works; I can see the benefit for some... but for most I bet regular cell minutes are still the way to go.
Someone please correct me if I'm wrong.