At the moment there are two competing plugins for Eclipse that can provide SVN integration, Subversive and Subclipse. At various times I have tried each of them and they both have their plus points however if I have a project checked out with one plugin and the Eclipse installation I am using doesn't have that plugin installed I don't get any options under the team menu to share the project.
Rather than getting the old plugin back running and "un-sharing" the project you can just delete a file from the workspace metadata to allow you to re-share the project.
workspace/.metadata/.plugins/org.eclipse.core.resources/.projects/{projectname}/.index.properties
This seems to work for me under Eclipse 3.3.2.
