Anyone else have issues with the new People Picker functionality in 2010? The People Picker control is used to select users, groups, and claims to grant permission to items such as lists, libraries, and sites. When you use the library permissions page to give users permission levels for the library, you use the People Picker control either to type user names and verify that the user accounts are valid, or to search for a name or partial string and return a list of users, groups, or claims that match the value you entered. When we upgraded from 2007 to 2010 some of our customized web parts that used People Picker suddenly started showing up blank.
It seems the only viable workaround is to build out a custom claims provider, which is outlined pretty well in the article HERE
Seriously Microsoft? Why break all this stuff?