According to X360Key, it's not compatible with all slim models.
If I bought a brand new Xbox 360 tomorrow, how will I know which drive it has, and whether it will be compatible or not.
I actually sent them an email, twice, but never heard from them.
What does that say about their support?

I don't know which drives are/aren't compatible, but you can use this list to make a pretty good guess based on the manufacturing date on the box.

http://www.360drives.com/list

Thanks for your reply, but how do you check the manufacture date without opening the box?
From the packaging and the box itself, how can you get any information on the Xbox 360 inside?

I don't remember if the slim boxes have it, but I remember on the phat boxes there would be a hole cutout so you could see the serial# and mfg date on the 360. For returns and whatnot.

The box doesn't show the manufacturing date, BUT, that list also has categories for "Lot Number" and "Team" and I'm looking at the outside of a Slim retail box that I had laying around. It shows the "Team" and the "Lot number" on the outside of the box, and you can use those to determine what drive it has. In my case, my team is "FDOU" and my lot number is "1138X", which indicates that I have a LiteOn 16D4S, and this is the drive I had in that Xbox when I still had it.