Oy, sorry ... I read your question incorrectly.
For Exchange 2007 you only need the mailbox role. BES doesn't need a CAS role to communicate with an Exchange 2007 mailbox server, as you can talk direct MAPI/RPC to that server.
For Exchange 2010 you'll need a CAS to communicate with a mailbox server as there is no direct MAPI/RPC communication with a mailbox server ... all goes through a CAS.
Sorry for the confusion.
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