Apple's MobileMe News blog is back after a bit of a hiatus with some helpful info on how, exactly, MobileMe and and Microsoft Exchange ActiveSync work and play together (or vice versa):
You can enter new information for each service separately as you move around, and that information is stored and synced separately. But when you check your contact or calendar information you can view the information from both jointly or separately as you choose. It's an approach that preserves the data integrity of each service while delivering the convenience of mobile access to both of them.
I use both MobileMe (for personal) and Exchange (for work) on my iPhone, and the above holds true for my experience so far. In fact, it lets me compartmentalize things nicely.
Anyone else two-timing on the push? How's it working for you?