[ale55andro]

You raise a point which is interesting if there was an official word from microsoft that this will be the last version. Right now this is still only speculation. Anyway, from a pragmatic view point SL5 is available today. A fresh release like this is a good reason to rejoice instead of speculating when the next release will be. Out of curiousity, what alternate are you suggesting that has the same unmatched tooling, documentation and support "today"? Specifically, if the mobile/tablet is not your target and your main audience is desktop on mac/windows and perhaps linux(note: with a small effort you can make your app work on moonlight too).

Edit: note also that with a small effort you can target the new windows 8 marketspace and release it as a metro style app as well.