No repository found containing: org.maven.ide.eclipse.editor.xml
Eclipse 3.4 brought us the p2 update manager. Besides some nice features and performance improvements it also brought many problems. One of them was the damned No repository found containing: xxx error message. I finally found a solution that even makes sense.
So here is the way to happiness: open site.xml, artifacts.xml and context.xml in your favourite browser and refresh them with Ctrl. + F5. Afterwards re-add the updatesite to eclipse and try again. Please note that I needed to do the refreshing for both updatesites of m2eclipse (http://m2eclipse.sonatype.org/update-dev/site.xml and http://m2eclipse.sonatype.org/update/site.xml)