I've tried to replicate this and cannot.
I'm running Windows Server 2008 Ent with Exchange 2007, and another similar machine running IIS7. For Webmail I am using Microsoft's OWA (I believe it came with Exchange 2007). For Outlook I use the one from Microsoft Office 2007, I run Microsoft updates daily on all three machines (2 servers and desktop).
What version of Exchange and OWA are you running?