Microsoft just released a Virtual PC image of XP and IE 6 so that web developers can continue to test with IE 6. 
Its a pre-activated version of Windows XP that is time bombed for April 1st 2007.

I guess you could always just take an extra key from your MSDN subscription and run it that way too and just not upgrade to IE 7.

Download here

Update:  I recently came across this site at Tredsoft that also allows for testing different browser versions side by side.