I bought my Xbox360 from Walmart.com about 2 months ago and I made sure to get their warrenty plan as well. But, let me put out a word of advise. The warrenty appears that you can only buy online, you may have to talk to a manager or somebody in store. Also, if you do buy it online, they won't send you any paperwork stating that you have a warrenty. So make sure you keep the receipt, and hope you stay in their database of names. And it appears that the warrenty plan isn't stuck to you xbox360, it's only price range, and it is transferable. But I wouldn't be abusing the system, but I think if something else you bought from Walmart broke, and it's in the 360 price range, you could use your warrenty on that (but then you probably couldn't use it on your 360 later on).
Finally, it appears to, at least how I read it, that the walmart warrenty doesn't kick into effect until after the manufactor's warrenty expires. With the new 3-year warrenty plan by Mircosoft, this could in theory bump up your system warrenty from Walmart to a 6-year plan (3 from Mircosoft, and up to 3 from Walmart). I haven't had to use my warrenty yet, since my 360 still works, so I can't say how well they will fix it if it does die. I assume, they will just swap it for another one on the shelf, but I can't be for certain.
Always read those warrenty plans very carefully. Don't just look at the length and the price, make sure you can get the services you want from it.