The thing is, a regular Skype call uses VoIP, meaning it uses the Internet connection for transmitting the voice data. The Skype calls you make with a device from Three on a prepaid basis, on the other hand, are not real Internet calls (i.e. they don't use VoIP), but they get routed as normal phone calls. So technically, you don't need a gprs-connection for that. What you do need it for, however, is for logging into your Skype-account from Three, so they can count the minutes and bill you once you used up your prepaid amount (which seems unlikely if you have 4000 mins a month). Bottom line is, I don't think you can get your N900's Skype to use those Skype-minutes included with the X-series, because it can only make VoIP calls. In order to log you into Sykpe and then make a normal phone call, it would have to be an special "Three-branded/tweaked" build of Skype. It might be possible if Three sold the N900 with an X-Series plan, but they don't, at least here in Austria. I also have my contract with Three (Austria though, not Australia), and I use the built-in Skype, but not with X-Series, but with an additional data plan for my phone, which is actually quite convenient.