I've been using the Docudesk utility to transfer .lrf's (and other files) to the Sony Reader on OS X (Tiger) and it works great. Their free (for OS X users) pdf->lrf conversion utility is handy also.
If you're comfortable with the Terminal/Unix command line, you'll find Kovid's libprs500 package for OS X many more powerful utilities to manipulate/convert your various files (pdf, txt, rtf, html, lit, web sites to Sony's lrf format, modifying metadata, etc). There is a GUI which is working fairly well now, but it's still work in progress. Make sure you copy the libprs500.app to your Home directory before you run it for the first time (it seems to like running from there only, at least for me it did).
One nice thing about the Docudesk utility is that when it is running, the Sony Reader is mounted in your /Volumes directory so you can copy/delete/view files directly on the Sony Reader (cannot edit files on the Sony Reader directly it seems, just make a copy to your local working directory, modify it there and then copy it back) as it uses the MacFuse kernel extensions (makes the Sony Reader's rather funky filesystem readable to OS X).