Initially I hypothesized that we could have an Exchange 2010 RC server with all roles installed. Then we would have our Exchange 2007 SP2 server with CAS and Mailbox role installed. Then we would have our BES 5.0 server. Since this current version of BES does not support Exchange 2010 the purpose of the Exchange 2007 SP2 server would be to communicate directly to the BES. However, after what you have said and GMCloud27's post saying he couldn't get his BES to accept the Exchange 2007 SP2 server as a mail server this work around wouldn't be possible.
Does that make more sense to you?