The update is of course a bug fixes and bringing the security improvements and so far only a few findings come out telling what is hidden in this release, but I believe soon will be more.
For example, the famous blog dedicated to covering Apple and its products, Macrumor found an interesting part from this Beta 3, which is 3G cellular data access's toggle is put back on its place.
And then on its third post update, Macrumors added with a new strike, it is a new API to allows developers flag app content that not supposed to be backed up. It will mark both files and directories like adding the attribute “For NSURL objects, add the NSURLIsExcludedFromBackupKey attribute to exclude the files from being include into backup.
That would be all from now, tomorrow or perhaps in next few hours there would another finding posted on the net informing the hidden treasures of iOs 5.1 b 3 brings to developers and to us eventually. Untill then, if you are a developer? Please procceed to this Apple developer site link and bring your own finding to us!