In the plugin development environment for Eclipse 3, you can see the source for some of the plugin library code via Navigate => Open Declaration. (By plugin library I mean the various org.eclipse packages plugin code might call.) And indeed if you look under Project => Properties => Java Build Path => Libraries, you'll see that the plugin wizard has set the proper source attachments for each of the plugin dependencies.
But you can't see the javadoc for the plugin dependencies via Navigate => Open External Javadoc. If you go back to the project properties, the javadoc location for the plugin dependencies is not filled in. I could fix this by hand, but there are at least 11 plugin dependencies, and I would have to do this separately for every plugin.