Adobe Labs has posted a release candidate update to Lightroom 4.2. The new release supports new cameras including the Canon Rebel T4i, adds new lens profiles, and fixes various bugs and issues. The update also now fixes tether support for the Nikon D4, D800, and D800E (win only). You can read more about the details here and download the update here.
A Release Candidate is a pre-release of the update to get user feedback and make sure it works all right before the official release.