The GUI for managing plugins in Eclipse got a bit of an overhaul in version 3.4.0.
This GUI is accessed via the \"Software Updates...\" option in the Help menu.
This displ
I'm running (a relatively fresh copy of) 3.4.1. I was able to select the same plug-in shown in your screen shot (Mylyn Bridge: Java Development) to get an enabled "Uninstall..." button. I'd suggest getting the latest updates and trying again.