Ok to the good stuff.
My build date was 5/29/2009 so I had part number 20840958 installed. The hardware version is CB/CC.
I have seen references to part number 20927509. This hardware version is DA/DX
I have also seen a new part number 20985421 in the catalog. This is version DB/DB.
This is not anything I've picked up off the internet. All 3 modules, versions and pictures were done by me today with my own personal vehicle.
If you don't have the PDIM and you buy the latest PDIM kit, 20940641, it will come with part number 20927509 (DA/DX). I know this because I just got a 20940641 kit on Friday and opened it and the part number on the module is 20940641.
It does appear to me that GM will be adding the new number, 20985421 to that kit, but it has not been done yet.
There is all of the good news.
The bad news is, if you have a 2010, the latest part number in any catalogs, with GM or reference material is 20927509 (DA/DX). If you get it done under warranty for a 2010 you will only be allowed to get 20927509 or the claim will bounce.
If you have a 2011 and you don't have part number 20985421, then you can get it and warranty will pay for it.
So the current PDIM add-on kit does not have the new part number in it as of this moment. I do believe, however, that it will shortly.
Personally, with the 2010's, I don't know why GM just doesn't change all of the part numbers over to the latest one, 20985421. The 2011 module works in the 2010 as that is what I did today. Any other questions, just ask. I'll answer if I know the answer!