According to http://www.assembla.com/wiki/show/scala-ide/Troubleshooting in order to have the Scala Eclipse plugin to work with eclipse 3.5 you must desintall your Spring-IDE pluging since the latter deactive the JDT Weaving. I had to do so.
Do you know any effective way to circumvent this, I'm an active Spring developer, for obvious reasons I also want to incorporate the Scala-IDE to my environment.