if you jailbreak an ATV2 , can you UN jailbreak it if you want to later on?
Restore back to original ??
Can you jailbreak if the ATV2 is at version 6.0 currently?
Yes.
When you jailbreak, you will disable updates so that a new firm/software release by Apple won't be downloaded, which would cause you to lose the jailbreak. So, if you want to "unjailbreak" later, just update your Apple TV to the latest firmware (Settings --> General --> "check for updates" on the Apple TV). This will update you to the latest firmware without the jailbreak.
Yes. You can always restore to factory settings, through the Apple TV settings menu.
Yes. Apple TV software 5.3 (iOS version 6.0.1, I think) is available as a tethered jailbreak.
http://support.firecore.com/entries/387605-Jailbreaking-101-Seas0nPass
Do you have to do a tethered boot when you are done?
Yep. As the link I posted above states, the latest jailbreak for the latest iOS software is "TETHERED". So you have to connect the Apple TV to your computer/iTunes when you reboot it for it to work.
I guess I have to tether it again if its un plugged or power failure? If so just do the tether step again from the same computer or do I have to JailBrake all over again?
Correct: if the device is ever powered down or rebooted, you need to connect it to the same computer running iTunes to complete the reboot. You do not have to jailbreak all over again.
BUT now I went into the menu and I see the FC Red icon in settings so I know it worked bit I dont see the XBMC icon !!! What did I do wrong?