I already blogged about the Exchange 2010 Hoster Edition… it is generally speaking not a good idea to try this in an enterprise environment… The Exchange 2010 Hoster Edition: http://sysadmin-talk.org/2010/09/address-list-segregation-or-hoster-edition-exchange-2010-sp1/
But currently I’m working for a hosting company, and I have to prepare a migration from Hosted Messaging and Collaboration (HMC4.5) to Exchange 2010 Hoster Edition. So, I have plenty of time to play with it… (And Greg, I have the SPLA licenses, don’t worry 😉