Apple has released a new build, referred to as 9B5117b, has been seeded for developers. This update is not available over-the-air as past developer builds have been.

This release includes enhanced dictation support during text input, but only on supported devices.  This leaves open the possibility that a the speech recognition portion of Siri may be added to iPod touch, iPad and earlier iPhone models.

It is important to note that once this version of iOS is installed that it can not be rolled back. iOS 5.1 requires the use of the newest Xcode 4.3 developer preview toolset for building third party apps.