It seems logical that in the thumbnail view that the private site would always be on the top left of the thumbnail grid rather than being pushed along to the right and down the rows every time you add a new public site. Because that private site is probably a special site that the user often wants to access, it would make sense not to have to scroll the page to find it. Also, I'd love to be able to rearrange those thumbnails whether or not you implement the previous request, so that I can keep the thumbnails for the sites I work on the most at the top.
Drag and drop feature would be cool though. But I just use "Last Modified" one