After upgrading osx to 10.6, it turned out that JVM 1.5 wasn't available anymore. This lead to GWT refusing to run in hosted mode, which I btw. think Google ought to fix.

As with most technical/computerrelated problems, the solution was found on the web:

http://wiki.oneswarm.org/index.php/OS_X_10.6_Snow_Leopard

Works like a charm; now I can get back to work..

I started out following the official guide (http://code.google.com/webtoolkit/gettingstarted.html), with a couple of additions:

The projectCreator script doesn't create a folder for the project. At least not when creating an eclipse project.
The default libstdc++ on Ubuntu Hardy is 6, while the gwt specifically requires 5. sudo apt-get install libstdc++5 fixed that.