Hosted mode on Mac broken

I have previously been able to use "hosted mode" perfectly for a long
time. Then, a while ago I did some kind of update, maybe of Eclipse,
and now it doesn't work any more. I keep getting the message

Development Mode requires the Google Web Toolkit Developer Plugin

No matter how many times I install this plugin, I still get the same
error message.
The list in Preferences -> Extensions is completely empty, and there
are no GWT items in the list of plug-ins.
Cross compilation still works.

I do have Firefox and the GWT extension in Firefox works fine. But I
want it working in Safari too.

The setup is as follows:
Mac OS X v.10.6.8
Safari Version 5.1 (6534.50)
Eclipse Platform 3.5.2
Google plugin for Eclipse 3.5
GWT SDK 2.3.0
WindowBuilder 1.1.0.r35, quite recently installed (possible culprit?)

Help?

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