Aahz,
Make sure that your log directory & associated logs are writable
(probably should also add a .htaccess to prevent anyone from seeing/
modifying as well).
Also check your server's standard logs...I found on my host that
rails raised the logging level to stderr which ended up in an error
log, but your host probably is different.
Several things to other things to check are the permissions on your
dispatch method (.cgi, .rb, .fcgi) and ensure they are 755.
-Dale