You can download 5.1 iOS beta 3 right now if you're a registered developer, from the Developers Portal. Apple has seeded iOS 5.1 beta 3 for iPhone, iPad and iPod touch to developers.
The first beta of iOS 5.1 was seeded to developers back in November 2011. At that time, the iPhone 4S was plagued with battery issues. It was thought that Apple will rush up the release of iOS 5.1 just to fix this problem once and for all. However, Apple later released a new iOS 5.0.1 build just for the iPhone 4S and that has since taken care of the battery drain issue.
Then in the first week of December, Apple seeded iOS 5.1 beta 2 to developers. Findings started coming in as soon as people started to download 5.1 iOS beta 2. This particular beta update brought a new, much sought after feature. In iOS 5.1 beta 2 Apple provided the ability to delete images from Photo Stream. Photo Stream is an iCloud based feature. If you have multiple Apple products that are using the same Apple ID, any pictures taken from any of these devices will automatically be added to the Photo Stream. This is then accessible through these very devices, no manual syncing is required. It just works.
Now, developers can download 5.1 iOS beta 3 for iPhone 4S, iPhone 4, iPhone 3GS, iPad 1, iPad 2, iPod touch 4G and iPod touch 3G from the Developers Portal. If you are already running iOS 5.1 beta 2 on your device, then this update can be downloaded over-the-air. The download weighs in at 76.2 MB and has been assigned build number 9B5141a. The release notes aren't extensive. It's only said that iOS 5.1 beta 3 comes with bug fixes and performance improvements.
Bear in mind that you have to be a registered developer in order to gain access to beta iOS firmware. You can only download 5.1 iOS beta 3 legally through the Developers Portal.