Started having this issue with all of our Outlook 2007 users when they go into Scheduling Assistant. I recently updated my exchange 2007 server with the rollup 7 patch and also updated the .NET frameworks on the server to 3.5 sp1. Ever since then when I go into Outlook and use scheduling assistant to set up a meeting it will lock up Outlook whenever I add a name to the meeting to see if they are available.
I can go into OWA and it will work just fine though.
I'm thinking it would have to do something with the windows patches that I just applied on Friday to the Exchange server that is running Windows 2003 Standard R2 SP 2.