Recently I received questions from our customers on SharePoint 2010 People Picker behavior when provisioning web applications in Claims mode using trusted identity provider such as AD FS 2.0, complaining the control not behaving as it does in Classic mode or in Windows Claims. I definitly recommend folks to review following newly published TechNet article when deploying SharePoint 2010 in Claims mode and consider if they need to plan for a custom claims provider before rolling out. Here is the snippet of the article pertaining to this topic:
"By default, when you use SAML token-based authentication, all queries entered in the text box are automatically displayed as if they had been resolved, regardless of whether they are valid users or groups. If your SharePoint Server 2010 solution will use SAML token-based authentication, you should plan to create a custom claims provider that will implement custom search, name resolution, and list features. For more information about custom claims providers, see Custom claims providers for People Picker (SharePoint Server 2010)."