Google now has available APIs to help migrate email from your Exchange server to Google apps for small business, enterprise, schools and personal use. Google Gmail, Calendar, Google Talk and Google Docs are of course the cause of Microsoft's response with Office Live On Demand applications.
Google is also trying to ease the transition to Gmail by supporting email delivery to both Gmail and Exchange accounts, run only a subset of accounts on Gmail, and offering Postini spam filtering and policy management for email.
The takers so far? It's hard to tell how many might have moved off of Microsoft Exchange and onto Google applications (I suspect not many mass transitions have occured yet) but both Gmail and Office Live make for compelling offerings for small business and those looking for more email and calendaring features than an ISP typically offers. (Here are more details if you are interested.)
I expect one of the most successful channels for these On Demand apps will be the IPS who chose to resell these solutions. In the end, both are upgrades for many businesses from POP3 and IMAP hosted email services.