geotag-lightroom-plugin is Jeff Barnes's plugin that adds GPS data to photos from within Lightroom. Locations are read from the standard GPX file format and exif information is stored in the image files using Phil Harvey's exiftool. Just tried it, and it seems to work, though it is a work in progress. Interestingly, it adds menu items to the standard Lightroom interface - hm, didn't know you could already do that.