I previously upgraded my Redmine install from 1.1.0 to 1.2.1 an needed to supply a mongrel.rb file in the config/initialization folder. This was needed because of special requirements for use with Mongrel 1.1.5.
After upgrading to 1.3.0, I copied my normal configuration.yml and database.yml to the config folder following the upgrade instructions, as well as the mongrel.rb to the config/initialization folder. I also updated Rails per the requirements for Redmine 1.3.0 to 2.3.14
I restarted the Redmine service and got a blank white screen. After doing a little digging, I realized that the mongrel.rb was missing the entry for Rails 2.3.14, so I added it, restarted and voila! Fixed.
An updated file can be found here.