MDS 4.1 will be a part of BB 4.1, which is scheduled to be released late this years
There are 3 components:
1. MDS Services - part of BES Server (not free, connectivity piece of server to corporate apps)
2 - Studio off BB website (free)
3 - MDS runtime environment component (free, app. part of 4.1 OS like Memopad)
I'd compare this as
very similar to AvantGo's enterprise software model (iAnywhere). Client app, with data that can sync and be stored on the BB, etc....
http://www.ianywhere.com/products/mobile_solutions.html