During Exchange Server 2007 schema extension you may encounter a security descriptor error which will cause setup to fail.
In some scenarios Exchange Server 2007 will not warn about active mailboxes when removing mailbox databases.
Here are some articles and blog posts that I've written elsewhere lately.
I've just drawn the winners of the GFI MailEssentials giveaway competition. Congratulations, and thanks to GFI Software for supplying the prizes.