Out of the box, SharePoint 2010 as of yet only supports SAML 1.1. This is bad news if you, for example, were planning to use claims authentication and let the AppFabric Access Control Service (ACS) handle authentication. What can you do? Basically, create a custom authentication provider, use ADFS, or use ACS SAML 1.1 translation. Here are some links:

MSDN / Stackflow