I've got a 3GS with the latest official firmware.
I'm very interested in using this MyWi app. I've got 2 questions.
1) If I don't want to jailbreak, is there any possible way to get MyWi working on my iPhone?
2) If I do jailbreak, is there any way to ever get myself back to the official firmware?
1. No
2. Yes, a restore
If you are on the latest firmware (3.1.3) on a 3GS, you are currently not able to jailbreak if you haven't already.