I use to use PFDavAdmin to export item count - here is a faster way to do this w/Ex2k7...
Download the Exchange 2007 Powershell Scriptacular demo pack and copy out-html.ps1 and out-ie.ps1 to a directory on a computer in which the Exchange Management Shell has been installed on.
Navigate to the folder w/in powershell you saved the out-html/out-ie.ps1 files, and run the following cmd.
Get-mailbox -server ExchangeServerNameHere| Get-MailboxFolderStatistics | where {$_.itemsInFolder -gt 5000} | sort-object -descending ItemsInFolder | select Identity,FolderPath,ItemsInFolder | .\out-html | .\out-ie
Update: I just want to point out that it may be necessary to use the –ResultSize unlimited switch with the Get-Mailbox command, otherwise you default to 1,000 items for Get-Mailbox-server.
Old way…
Using PFDAVAdmin to get the item count within the folders on mailboxes on an exchange server
http://msexchangeteam.com/archive/2007/04/24/438170.aspx
Understanding the Performance Impact of High Item Counts and Restricted Views
http://technet.microsoft.com/en-us/library/cc535025(EXCHG.80).aspx