I just updated this site's configuration to use Mongrel on Lighttpd. It's not really a recommended setup, but I'm not using any of the features that are reported as being problematic.
I haven't noticed a marked increase in speed over FCGI, but I do believe it's now able to withstand a larger pounding. I'm planning on trying to hack together the Lua-based magnet caching that the above link describes, which will allow lighttpd to completely bypass the mongrels to serve cached Rails content.
This server is running Gentoo, and thus I have a love/hate relationship with it. I mostly love it, but right now I can't figure out how to do what I want to do: install Ruby 1.9 on Gentoo. It's been reported (both sensationally with little relevance and with relevant comparisons — Ruby 1.9 is listed as 'yarv' in the second benchmark) that this new version is significantly faster than 1.8, so I'm excited to try it out in the wild. Unfortunately, I haven't found any good instructions for installing it, and my Gentoo-foo is weak. I could easily download the source, compile it, and symlink the binary... but I really don't want to clutter my server up with an intricate maze of symlinks that only I know about. No, I'll wait until I can install 1.9 with Portage (which may be now... if you have insight let me know).