Unlock, Root, and Return to Stock
To unlock your phone - that is allow you to put another carrier's sim in your phone, check out android.jakaar.net/infuse-unlock-sim.html
The above method to unlock the phone does NOT require your phone to be rooted. The method will use a shell root (temporary) to acquire the unlock code.
If you want to root the phone, check out this page: android.jakaar.net/infuse-rooting.html
If you want to undo your root - that is make the phone completely stock again, you can always flash it back with Odin. Go here:
Or, you can download the SuperOneClick and click the "Unroot" button if all you want to do is unroot the phone. The link above for how to root your phone will contain a download link for SuperOneClick