I haven't been able to get IE9 (x64) working with OWA 2003 running against an (x32) SBS 2003 Server. There doesn't seem to be any back end updates to make it work.

So I'm un-installing IE9 (Despite it being the most standards compliant IE ever)

When you go to uninstall IE9:
Don't
Control Panel ==> Programs ==> Programs and Features ==> Turn Windows features on or off ==> Uncheck Internet Exploerer 9 ==> Reboot! (this does disable IE9 but you don't get IE8 Back)

Do
Control Panel ==> Programs ==> Programs and Features ==> View Installed updates ==> Select Internet Explorer 9 and click uninstall ==> Reboot!