I just had the same problem about a week and a half ago I talked to microsoft and they sent me a box to send my unit in they told me about 7-10 working days I should get my xbox360 back by the way mine was a launch unit they did'nt even ask about when I bought it they just said they would make sure and fix my xbox360 or send me a new one now thats taking care of the customer in my book.
That's a run-on sentence if I've ever seen one!
I'm glad the process is smooth... do they end up giving you a brand new system? I would hate to run into similar problems again in the near future. Also, is there a warranty available for purchase through MS?