The final release of Adobe Lightroom 4 is now available for purchase or 30 day trial. All the important details are on Tom Hogarty's blog. Several items of note:
Congratulations to the Lightroom development crew, this is a great piece of work! I believe this is a must have update for interior and real estate photographers just because of the improvements in the new Highlight and Shadow controls in the Development module.
Update: 3/6: Be sure you check the system requirements against the hardware and OS that you have. LR 4 doesn't run on Win XP or Mac OS X versions before 10.6.8.
Update: 3/8/2012: There are a bunch of questions below about Plug-ins etc. The fastest place to get answers on these issues and see what problems others are having is to take a look at the Adobe LR 5 forum.