Sent to you by Chris Hunter via Google Reader:
If your iPhone is feeling a bit sluggish after recent carrier updates from AT&T or T-Mobile, iTweakiOS has a guide for installing your own custom carrier bundle that should bring increased speeds.
Every once in a while, your cell phone carrier pushes out an update to you phone that changes the preferences so it can find the best signal. However, that's not always the fastest signal, and in the case of a couple recent updates on both AT&T and T-Mobile, those updates might have actually slowed down your connection a bit. iTweakiOS put together a custom carrier bundle that overrides the official one and forces your phone to hunt down the fastest signal. The new carrier bundle works on the iPhone 5 on T-Mobile. It also works on the iPhone 5, iPad, and iPad Mini on AT&T.
You don't have to be jailbroken to use this and the process happens in entirely in iTunes, so it's not too difficult. That said, we didn't have a chance to test it, so proceed at your at own risk. Head over to iTweakiOS for the full guide.