Hi Jon,
Love the work on the new ClientAPI, it's looking awesome.
I just noticed something weird on the webcontrols site that I wanted to bring to your attention. I tried to search if it had been raised before but the standard DNN forums are pretty horrible to work with tbh.
Have a look at http://webcontrols.dotnetnuke.com/samples/TreeCheckBoxMode.aspx. If you set the CheckBoxMode to SingleSelect, you will find that it correctly does the removal of the currently checked boxes when you check one of the others. However, if you click on one of the leaf labels, it also checks the box for that leaf, which doesn't seem to do the clearing bit first. This results in the ability to select as many checkboxes as you would like.
Another weirdity is clicking on the Help->About node, it checks the checkbox and also shows some popup message (/ t seem right to me :)
Cheers,
Max