I word in California for IBM and we have a use it or lose it plan. The way they account for this is this: In January I get 3 weeks of vacation. The following January I get 3 more weeks minus whatever vacation I had left. That is, my original 3 weeks never expire, but if I don't use it by January I don't get more vacation.

You're correct, MS supports this with exchange 2007.

However, google has not leveraged this yet. Push gmail (without going through another server) is still on my wishlist.

Push e-mail is still not available (at least for a WM phone).