Christian,
Yes we are advocating larger mailboxes due to the architectural changes we made in ESE and Store as a result of moving to the 64-bit platform. One of those changes is an increase in the number of storage groups (from 4 in E2K3 to 50). Also you can have 50 databases per server (we are still keeping hte SG database depth at 5). So today you might have 20 databases with 250 200MB mailboxes and that potential db size might be 50GB. Well in Exchange 2007, you can have 50 databases, so you could have 100 mailboxes that have 500MB size limits. Or you could go higher on the mailbox size limits and reduce the number of users (to ensure your backup strategy is met). Also remember that we also have the continuous replication strategies which will reduce the need to backup against the production data (you will be able to utilize VSS to backup the replica).
Basically it all comes down to your requirements, SLAs, RTOs and RPOs. They will still drive the underlying design.
Microsoft
