Apple has released iOS 6.1 beta 1 for iPhone, iPod touch, and iPad. It's available to paid developers via developer.apple.com and carries build number 10B5095f. The iPhone version is over 1GB, so plan your bandwidth accordingly.
Speaking of iPhone versions, the iPhone 5 versions are listed by model number, so make sure to check Apple's iPhone 5 model number page to make sure you grab the right one.
Also, if you have a new iPhone 5, or you're getting a new iPad 4 or iPad mini, make sure you add the UDIDs to Apple's developer portal before you update (or you'll have to dig through Xcode, system profiler, or third-party deployment tools to find it.)