BES 5.0 supports an Exchange Resource Forest configuration.
Good article on Exchange resource forest config Deploy Exchange 2010 in an Exchange Resource Forest Topology: Exchange 2010 Help
Multiple AD forests should be ok as long as
1) There is a two way trust between them
2) They are all within the same Exchange organization
A new exchange org means wipe the device prior to activate on new server unless you use a service like B nator