I don't think installing the Beta of 4.0 is going to allow you to run Skype in the background. That's going to require a new 4.0-compatible version of Skype (to use the new VOIP multi-tasking API's) which won't be available until the official 4.0 release is here (and given Skype's track record with updates -* where's VOIP over 3G *- it might be much later than that). Same thing goes for any other app you want running in the background (Pandora, GPS, etc.). Even for the quick state saving that regular apps (e.g. some random game) are supposed to do when backgrounded, I think you need new versions, so wouldn't expect MT to work right even for those.
What I gathered from the keynote was that aside from apps using the special APIs (for VOIP, GPS, audio streaming), what would really happen when you MT is that each app would do a complete and total state save, then be terminated, then quickly relaunched with its state exactly restored when you flip back to it. So you "think" it's MT-ing and running in the background but it's really not. Kind of like hibernating a Windows PC. (I could be wrong about this though- I haven't had time to dig into the SDK's release notes yet.)