Lightroom 2.0 was officially released today. The US$99 update is available immediately via Web download. Rather than try to repeat all the new features and changes in Lightroom 2.0, I'll just send you to Adobe's own Lightroom Journal page for all the details (and there are lots!).
Short version: Adobe made many welcome UI changes that make Lightroom even more direct, the new Adjustment Brush and Gradient Filter abilities are nice additions, multiple monitors are supported, and lots of little issues have been tackled (vignette works on the crop, files can be opened as Smart Objects in Photoshop, etc.). As Scott Kelby points out, there's still more to be done and Collections took a step backwards (I'd add the whole Catalog thing is still not quite right for the traveling photographer).