The latest update brings fast-app switching and multitasking, allowing the program to run in the background. This has been a long time coming for many users. The update is free, and so is the app if you don’t have it yet. There are some bugs, like push notifications putting any call on hold, but for the most part it’s a solid update
On a side note: to use FaceTime over 3G, check out “Fring”.