This is a question that I have been hearing a lot lately and thankfully Jeff Tranberry, Product Manager in Digital Imaging focusing on Customer Advocacy, has posted the excellent information here.
If you are looking specifically for support fort the Canon EOS 5D Mk III you can download the Lightroom 4.1 Release Candidate from Adobe Labs.

Is it possible to update Photoshop CS6 RC to support 5D Mark III too?
Tom Hogarty has posted where we are any why on the Lightroom Journal Blog: http://blogs.adobe.com/lightroomjournal/2012/03/raw-support-in-lightroom-and-camera-raw.html
Thank you. This link was helpful.