We recently went from Exchange 2003 to Exchange 2007 and my Sprint HTC Touch Pro ( stock wm6.1) stopped syncing with exchange.
Recently I flashed my phone to WM6.5 and it all started working again.
I believe this has something to do with the stock security certificates.
One place to start looking is here:
http://technet.microsoft.com/en-us/l.../cc182295.aspx
Hope that helps, the other option is to upgrade to WM6.5