I attended a Lightroom 3 presentation at the LA Web Development monthly meeting, and I had more doubts than answers then. I have been using photoshop since the days of Photoshop 7, and didn't understand why someone would switch from one to the other...
In one of my classes at UCLA, my instructor George Simian, had a lab day with Lightroom 3.3 and it all makes so much sense now...
If you are a person that loves to take photos and takes as many as possible, then Lightroom 3.3 is for you. It has advantages over Picasa and iPhoto, because Adobe has integrated many cool features to help with the organization, archiving, versioning and preservation of your RAW images.
Here are some useful links:
http://www.dpreview.com/news/1006/10060801adobelightroom3.asp
http://forums.thoughtsmedia.com/f314/six-reasons-why-adobe-lightroom-3-should-your-digital-toolkit-100984.html