When I went to SIGGRAPH 2007, I asked one of the Google engineers if there were any easy way to export files from SketchUp to Second Life. While I was standing there, someone else came up and asked the same question.
Clearly there's interest in some sort of plug-in, but there seemed to be very little interest from Google in writing it.
There have been a couple of attempts at creating a Ruby plug-in, but neither have produced results. Roo Reynolds claimed to have written a plug-in in 2006, but he never publicly released it, and it appears to be an abandoned project. Thomas Gründer was also working on an exporter, but as of this writing, the project has not been updated since August of 2007.
Meanwhile, you can export SketchUp Pro files in .obj format and convert them to Second Life files with a variety of tools. It's not a one-step process.
Update: Evgeni Sergeev wrote a Ruby script called Sketchlife. Sketchlife is sold as a cost-per-upload service within Second Life. You need to both download the script for SketchUp and have a scripted object within Second Life. You pay the in-world Sketchlife Importer in Lindens for each upload. It's still not a one-step process, but it's progress.