The iOS 5.1 beta 1 SDK is now available for download by paid devs at the iOS developer site. Sign in with your developer credentials to gain access to the updated SDK. You will need to download and install firmware via iTunes.
Because it cannot be said too often, make sure you read any release notes on-site. For everyone intoxicated by new technologies that might allow you to update using other means, I cannot strongly encourage you enough to go to the site and read the release notes. Installation alerts appear in a big yellow box on the main iOS dev page. The yellow box for 5.1 is larger than ever.
The beta is, as always, released under the terms of Apple’s NDA.