Well after a relatively short beta period, Adobe Lightroom V4 has been released.
Lots of information and training is now becoming available so I would advise you to check out some of the free on line tutorials and if your going to purchase V4 and want to make the best of it, I can recommend either the video tutorials from either Kelby Training or The Luminous Landscape.
For version 2 and 3 I upgraded immediately as there was functionality I really wanted to use straight away. Now I think I may wait. Last time when Adobe Photoshop CS5 was released they did a double upgrade offer that made the Lightroom upgrade a lot cheaper. Hopefully they will do the same this year.