Yes...you can always do a restore in iTunes...therefore erasing a JB should you need to return your iPhone to Apple/AT&T.
I don't know if it was clearly stated but you are better served to save your shsh blobs with TinyUmbrella prior to a JB. That way you can maintain your current baseband (maintains abilities to unlock) in addition to being able to revert back to a previous OS...if you need to.
If your trying to JB to OS 4.1...and you're are at 4.01 or 4.02...the procedure would be:
1) Save your shsh blobs with TU (if you wish).
2) Upgrade your OS ("Check For Upgrade") in iTunes 10. If you choose to upgrade you will be upgraded to 4.1.
3) JB with greenpois0n, limera1n or PwnageTool. Either will do.