Little is known so far about the latest beta, which was issued to test devices as an over-the-air update. iOS 5.1 beta 1 is not yet available to developers as a direct download from Apple, and as such, there aren't yet any release notes.
The second beta of iOS 5.1 came a month ago, and gave developers the ability to delete individual Photo Stream pictures. In current, public versions of iOS, users cannot delete individual Photo Stream pictures, and must turn off the Photo Stream feature entirely if they do not want a picture to be stored on Apple's servers.
iOS 5.1 beta 3 is compatible with the iPhone 4S, iPhone 4, iPhone 3GS, iPad 2, iPad, and third- and fourth-generation iPod touch.
The first beta of iOS 5.1 was provided to developers in late November. It included references to a next-generation iPhone, labeled "iPhone5,1," as well as third-generation iPad models, "iPad3,2" and "iPad3,3."