This seems to be a sporadic problem but is affecting FF, IE and Chrome. It seemed to work flawlessly when I first started adding Virtual Pages, but now that I have 50 odd, it seems to fail more often than work.
The problem happens when I load a Virtual Page in the CMS. There is a link on the main tab that says "click here to edit the content". This link should take the CMS user to the original page so they may edit the content.
Most of the time now, this link does nothing except display the "Loading" message at the bottom of the page.
I have monitored the action in Firebug, and see http://www.domain.com/admin/getitem?ID=52&ajax=1 working there with a final 200 OK response. I can confirm that the ID in these URLs is always the ID for the original page, so no problem there. The main content page just stays on the Virtual Page.
UPDATE: I have found that for this link to work, the original page MUST be visible in the tree. If it is inside a tree folder, which is collapsed, the link does not work.