Microsoft’s hosted messaging solution is still running on Exchange 2007 and it will be upgraded to 2010 only towards the end of 2010. Microsoft website states that hosting on Exchange 2010 will only be supported approximately twelve months after the release of Exchange 2010. Exchange Product Manager confirms the same here
The only cloud solution from Microsoft running Exchange 2010 is the Live@edu program, which is for education customers. Handy information for Exchange Admins!