If you purchased Adobe Lightroom 3 recently (many of you took advantage of the great Lightroom 3 deals), you may qualify for a free upgrade to Adobe Photoshop Lightroom 4.
I went in circles with Adobe customer support a few times to find out what the cutoff date was - and Feb 5th, 2012 was the answer. I'm not 100% confident in this date, but it is likely correct.
To get your free upgrade, follow the steps outlined on Adobe's Upgrade policy after product announcement page.