Here's a step-by-step guide on how to install the Google plugin on Eclipse Juno (release candidate).
It can also be found at https://plus.google.com/110140945975184013033/posts/ePFpQZYhPQq
How to Install GWT Plugin for Eclipse on 3.8 or Juno:
1. Download zipped update site for 3.7
from http://code.google.com/eclipse/docs/install-from-zip.html
2. Unzip content.jar; rm content.jar
3. Edit content.xml; change
<required namespace='org.eclipse.equinox.p2.iu'
name='org.eclipse.platform.feature.group' range='[3.7.0,3.8.0)'/>
to
<required namespace='org.eclipse.equinox.p2.iu'
name='org.eclipse.platform.feature.group' range='3.7.0'/>
4. jar cMf content.jar content.xml (zip won't work)
5. Install from local update site
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