@quipper: there are paid apps on cydia. afaik cydia have implemented their own apps license registry and payment system (paypal capable), so you just need to relogin to cydia on your freshly (blank) jailbroken iDevice to redownload all the apps you've already paid for for free.
There are also utilities to make a list of all the currently installed packages, so that you can mass-install those packages again on blank device on one go.

@bec: If you don't need to jump in/out of IM, then that limitation won't be much of a bother then. The remote-proxy system is useful for when you're in areas with bad/limited connectivity. It keeps you connected (maybe with 'away' status) and keeps receiving the messages even when you're disconnected. Useful for messenger service without offline-msg-receiving feature, like gtalk.

Shared file repository isn't available yet in OS4. I think they only allow you to send pictures from the photo library. I think you've to resort to email transfers for most of file exchanges.