Apple has released iOS 6.1 beta 3 to developers. If you're a paid iOS developer, head on over to Settings > General > Software Update to update in place, or developer.apple.com to download and install manually via iTunes.
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.