Exchange 2013 SP1: OWA native support for ADFS!
It's been a long time coming but we finally have native support for ADFS authentication for OWA and ECP. Native means no more hacking away at the web.config , messing with fedutil etc.. etc..
It's all built into two commands:
-Set-OrganizationConfig --> set the token signing cert, ADFS issuer and AudienceURIs
-Set-EcpVirtualDirectory and Set-OWAVirtualDirectory -AdfsAuthentication
See https://technet.microsoft.com/en-us/library/dn635116(v=exchg.150).aspx for more details