Satya's blog - Youtube on Roku
Assuming you have a local web server.
Step 1. Get Roksbox
Step 2. Go to Youtube, login, create a playlist (as many as you want).
Step 3. Get the alphanumeric ID of the playlist.
Step 4. Go to http://www.thesatya.com/cgi-bin/rry/get_playlists.pl and plugin the ID.
Step 5. Add the resulting URL to the roksbox videoqueue.xml file. (See roksbox docs for info.)
Step 6. Add whatever you want to the playlist.
Step 7. Enjoy!
From that link, you can also get the code.