I have been happily using Outlook 2010 now for a few months and love the access to my Exchange 2010 online archive. However, since SCCM is not functional yet some of my co-workers are still on Office 2007 (and thus Outlook 2007). Support for Exchange 2010 online archive has been announced a while back and I was under the impression that it was already implemented.
It seems you have to request and install the following hotfix: KB2412273
The added support for the Exchange 2010 online archive isn’t mentioned in the release notes, but trust me… it’s there!