BIS should work with Exchange 2007. The issue might be having a self-signed certificate you might need to purchase an SSL certificate from a publicly trusted certification authority for a Blackberry to connect through OWA.
Have you definitely tried accessing your Exchange OWA externally and not just from within your network?
Put in the OWA address on BIS, then try combinations of:
username: username or domain\username
mailboxname: username or blank
What errors do you get?