Andrew, app/assets/fonts is actually already in your asset load path, along with images. So you can just point to the asset in the same way: <%= asset_path('/Ubuntu/Ubuntu-R-webfont.eot') %>[1] or how ever you are referencing your images.
It took me a while to wrap my head around this as well. I still don’t know what happens if there’s a file with the same name in app/assets/fonts and app/assets/images.
[1] Assuming you have a font at app/assets/fonts/Ubuntu/Ubuntu-R-webfont.eot