I've been all over the internet looking for solutions.
Check around other posts for info on using Jeffrey Friedl's Light Room Plug in.
Also, just today, I found a site that claims to have a plugin that makes it easy to migrate from Picasa to Lightroom keeping all the tags you setup in Picasa. The website is picasa-lightroom.com.
I haven't even installed Lightroom yet. but am considering investing a few days to do so and test both solutions.
There's very little feedback or discussion on which of these work or how well. If you try one of these plugins and it works, I'd appreciate hearing about it.