I'm using 1.1.5 but this appears to still be an issue in master. Apparently the ActionController has been changed heavily in newer versions of rails and no longer has a AbstractRequest class. I'm currently using 2.3.4. I have tested and this can be fixed with the following:
- ActionController::AbstractRequest.relative_url_root = ops[:prefix] if ops[:prefix]
+ ActionController::Base.relative_url_root = ops[:prefix] if ops[:prefix]
in lib/mongrel/rails.rb