I've found creating Ant builds really counter productive thing. When I want to quickly test something by creating small new project linked to main project but also able to compile separately for quick test in real browser - that's somethign that starts to be time consuming, boring experience. That's my personal point of view because I'm not long-dated Java programmer and I have always used IDE features (NetBeans/Eclipse) to get things running without taking care about build internals.
CypalStudio has that feature built-in for regular use (as clean target/export to war implementation) in addition to standard hosted mode run/compile target:http://www.cypal.in/studiodocs
having both targets (run in hosted mode and generate output) was usefull for me recently when testing on ipod touch: i could test different versions at the same time using gwt hosted server instance running current project and using Aptana IDE local server to host output (compiled) files on different port. Both are usefull to run on real mobile device.
This would be nice if output folder can be specified the same way as in Run target wizard using either relative or absolute paths that can be outside of project root. That way compiled ouptut can be automatically used in other kind of projects (say with Aptana IDE plugins project opened in different or the same Eclipse workspace),