At the Exchange Server 2007 TechCenter, there is an article called Exchange Server 2007 Deployment: 10 Tips When Installing. Microsoft has tried to make the Exchange Server 2007 installation experience as easy and flexible as possible. They have largely succeeded at doing so. This does not mean you can just go ahead and run setup without thinking so read up on the technology, test it and plan, plan, plan...

From the article, a couple of things are interesting:

• You always need to install the Hub Transport role. Every mail that is sent will pass through this server. It makes journaling, rules, building ethical walls etc... much easier. In most deployments, you will put the Hub Transport and Mailbox role together on one server.
• The cluster installation process is integrated in setup. You get a checkbox in the installation wizard for the active node and one for the passive. For an overview on how to setup CCR with two nodes, check out my tutorial based on beta 2. Also check out the videos on the Exchange Team's blog.
• Exchange Server 2007 setup can be delegated to another user.
• Think about routing groups in the scenario where Exchange Server 2003 and 2007 coexist. Some more information about this can also be found here.

Check out the complete article, it is well worth the read.

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.