Hi David Bows,
This problem is actually caused by MS AJAX, on top of which all Prometheus controls are built. The MS AJAX engine is not so advanced as to register <link> CSS tags coming from an AJAX request.
Thus, the Input skin is simply not registered and lost.
To overcome the problem (for which there is not another solution) you should manually add a reference to the Input skin to the <head> of the page, e.g.:<link rel="stylesheet" type="text/css" href="RadControls/Input/Skins/Telerik/Input.css"></link>
the Telerik team