When you set up federation with AD FS (presumably Geneva), will it be possible to use Outlook as an active federation client, or will authentication with AD FS only be available with OWA?
Looks like only OWA is the client which can authenticate in ADFS environment right now. I am not aware of any method to configure it for Outlook but when I read this http://technet.microsoft.com/en-us/library/bb124507(EXCHG.149).aspx it clearly mentions OWA as the only client. You may want to wait for other comments on this post.
The OWA support was all official info I could find as well. On the other hand, if Geneva and Exchange 2010 is the platform for Microsoft's Online Services 2010/BPOS, I would reckon that to provide Software+Services, they would have to support a federated method for authenticating Outlook to the cloud based Exchange service.
To rephrase (maybe wrong forum, but it has to do with Exchange functionality):
Won't there be single sign-on for Outlook with Exchange Online? In the current version of BPOS, you have to use SSO client, but they've promised that it won't be necessary in the next version, due to AD FS support.