There used to be a copy-pasteable URL in the DevMode log window of the app you started (removed in 6559), I found this hugely helpfull when i was new to devmode and could not remember the the ?gwt.codesrv or the magical path/directory it put the project in (altho it seems to always be just the root / now).
-- http://groups.google.com/group/Google-Web-Toolkit-Contributors
