I did have my x-box mess up a year or two ago and had to send it in to get it fixed. I don't know exactly what they do as far as updated parts or anything, but I sent it in in the original packaging. Also, don't send any hard drives, controllers, memory cards etc. in as you won't get them back.

Once I got the email that my xbox had arrived it was only about a week, maybe a week and few days, before I had it back and working properly. I don't know if this is helpful or not, but it was pretty straightforward and over pretty quickly in my case.