I actually had the exact same problem. I am actually new to XMBC but not new to Jailbreaking. After purchasing 2 ATV2s we used them for a bit then they just collected dust. Now we are converting our WMC with Xbox Media Extenders into an XMBC system. Now we had a use for the ATV2s. But lone and behold, someone in the home upgraded both of them and they were running 6.0.1. Frusterated, I tried everything. Now I figured our how you do it.
Step 1) Download Seas0nPass
Step 2) Assuming you are running Windows; uninstall iTunes and restart your computer
Step 3) Download and reinstall iTunes 10.7 from OldApps (its free of any spam or freeware)
Step 4) Restart your computer
Step 5) Open Seas0nPass
Step 6) Click “Create IPSW”
Step 7) Follow the steps on the screen EXACTLY–//I cannot stress to you how important this is. I found one blog that said that you must never use the power adapter when using Seas0nPass–they are wrong! This is where I kept messing up, in that I wouldn’t plug in the power cord when it told me to.
//–IF SUCCESSFULL AND STILL NOT BOOTING;
Step 8) If Successfull screen appears, try to boot, if the ATV will not boot unplug it and relaunch Seas0nPass
Step 9) Click “Boot Tethered” from Seas0nPass screen; again FOLLOW ALL DIRECTIONS
-----IT IS VERY IMPORTANT THAT WHEN IT SAYS “Its safe to plug in the HDMI cable” that you do this quickly; DO NOT REMOVE POWER; ONLY USB
---------Basically in Boot Tethered; all your doing is replacing the USB with the HDMI; QUICKLY, when told to by the program
I hope this helps as a basic guide. If you have any questions about how I got it working, feel free to message me.