The long-in-beta iOS 5.1 software has a Gold Master (GM) candidate, build 9B176, and has passed quality assurance (QA) testing by Apple and their carrier partners. According to Will strafach of Chronic Wire:
The build number of iOS 5.1 Gold Master is 9B176 according to a very solid source, although three different partners who are testing the Gold Master claim to have slightly (by single digits) higher builds numbers on their copies, I'm assuming that's because right now it'd make sense for those people to be testing what will soon be released as 5.1.1, so I've disregarded the claims.
Currently, only Photo Stream photo deletion, faster camera access, Japanese Siri support, and some bug fixes, are rumored to be included in iOS 5.1, but Apple often keeps major new features under wraps until they debut with a new device.
Since tomorrow is the iPad 3/iPad HD event, it seems likely the rest of the features will be revealed and the Gold Master will be given to developers, with release a couple days before the new hardware hits the shelves.
Source: Chronic Wire