I am not 100% sure of my exposure but I am running Exchange 2000 and Exchange 2007 with Exchange 2003 on the BES (4.1.6) when I introduced Exchange 2007 I did nothing other than ensure that the access permissions were properly applied to the E2K7 boxes. A few thousand exchange mailboxes a couple hundred devices and so far so good. but a little worried......
__________________ 4 x BES 4.1.6
E2K & E2K7