grails run-appthen either open a second terminal or background the process (Ctrl-Z in a bash shell) then use
grails -Dselenium.remote=true test-app other:selenium <test name>to run individual tests without stopping or re-starting the app. With the app running development mode you can effectively test-drive using Selenium tests and Grails' artefact reloading capabilities.
One thing I should clarify is that direct domain class access will not work in remote mode. I'm thinking about ways to add fixtures support to the Selenium RC plugin so there's a good alternative approach that will work in remote mode.