Whenever I get a funny error message in my work, I Google for it - and this one didn't give me much that was helpful. So hopefully if anybody else gets this error they'll find this post.
I've deployed a new app for a client on my Rails machine and suddenly the fastcgi server would crash on startup with the error (slightly obfuscated here):
'execve failed for: [path]/XXX/public/dispatch.fcgi No such file or directory'
The main problem was that the shebang at the top of /public/dispatch.fcgi was pointing to the new OS X 10.5 'frameworks' Ruby rather than the more standard /usr/bin/ruby.