I have been using Mongrel successfully with rails 2.* and 3.0* development, with ruby 1.8.7.
I recently started working with Rails 3.1 and ruby 1.9.2. I got my test app running with WEBrick. I don't like WEBrick. If I forget and simply close the WEBrick terminal window instead of going into the window and issuing a Control-C to WEBrick, the server port (3000) stays in use, and I can't run 'rails server' again until I log out everything and get WEBrick cleared out of the port table. Mongrel never had that problem.
I do have a build problem with Mongrel and ruby-1.9.2. I get multiple header files in the build, some referring to ruby-1.9.1 and some ruby-1.9.2. What a mess.
What is the recommended development web server for my config, which is 32-bit Ubuntu Natty with Rails 3.1 and ruby 1.9.2?