I recently installed a Windows XP virtual machine under MacOS using virtualbox, only to discover that Outlook 2007 would hang when opened (unable to connect to exchange). It seems the issue has been documented in the following ticket but no resolution has been posted. Outlook 2003 seems to works without a problem.
Update: I’ve discovered that Outlook 2007 works fine if you switch the networking options in virtualbox from “NAT” to “Bridged”. It also fixed some other issues I was having (mapped network drives and group policy didn’t get applied. For reference the domain controller is a Windows 2008 SBS server.