Recent question across my mailbox: "How can I get a count of how many mailboxes are on each of my mailbox databases?"
The simple answer is to use something like this:
Get-Mailbox -ResultSize Unlimited | Group-Object Database | Format-Table Name, Count
You can refine it further (with -Server parameter or some other Filter details), if you like. Thanks to the Exchange 2007 TAP customers for coming up with this easy solution!