1) You can install appstore apps larger than 10mb over the 3g
2) You can use Voice over IP apps such as Fring on 3G! (Think free phone calls, no minutes, everywhere you have 3G).

By default, his app only activates itself when you load Fring or Appstore.

From the developer:
This MobileSubstrate library can trick any application into thinking that it is on Wifi even though it is on a cellular data connection. There are a lot of possible uses for that, but for me the main use is being able to use VoIP programs like Fring over 3G. You can configure what apps get tricked by adding the app’s bundle identifier to “/Library/MobileSubstrate/DynamicLibraries/VoIPover3G.plist”. Currently, the default is to trick only appstore and fring. If you remove or rename the VoIPover3G.plist ALL applications will be tricked. This is not recommended though, because for a lot of apps its makes sense to know whether they are on wifi or 3G.

If you use it for VoIP, it might be that your internet provider blocks the ports that are needed for the application to work. This cannot be circumvented by this library.

Note: If your provider forbids VoIP and catches you and you have to pay for it, I am not responsible!

This is cool enough that I am adding it to the ongoing “reasons to jailbreak” list.

Update (10/25/2008): Just released an update that adds iTunes store and Truphone by default. Truphone is another VoIP client so if Fring doesn’t work for you, you can try this one.