Microsoft has posted details about the available editions of Exchange Server 2007. You can find them here. The Exchange Server 2007 Standard Edition is interesting because Microsoft have removed some limitations when compared to Exchange Server 2003. The most notable changes are:

• Support for five mailbox databases and storage groups (only one in Exchange 2003).
• Unlimited database sizes (16GB in Exchange Server 2003 pre-SP2, 75GB with SP2)

Besides that, the Standard Edition also supports local continuous replication (LCR). With LCR, you can have a copy of your Exchange databases on separate local storage.

If you want clustering or cluster continuous replication (CCR), you will need the Enterprise Edition. That edition also supports up to 50 mailbox databases and storage groups.

There are some changes to the CALs as well. Check the web page for more info.