I've run into this problem a few times myself as I have been playing with the new Conceptual Energy Analysis tool as part of the Subscription Advantage Pack for Revit 2011. The delete button is grayed out and not selectable for projects in the Results and Compare dialog box.
Conceptual Energy Analysis stores project in the cloud, but also on your computer.
C:\Documents and Settings\%username%\Local Settings\Application Data\Autodesk\Revit\<Autodesk Revit Version>\CEA\DB
Within those folders there will be a folder for each project you have submitted. I've found that sometimes the read only flag gets set on those folders (randomly it seems).
To delete the projects you want to remove from the dialog, shutdown Revit and navigate to the folder above for your OS and delete the project folder and then launch Revit again, the project should be removed.
I'd be intersted in hearing if anyone else is encountering this issue, let me know in the comments.