When disabling a raddatetimepicker the width will change from its preset width to a much larger width when you hover over it. Putting the following code and viewing it in ie9 will show the issue.
I am using 2012.1.411
There is a workaround for this, I received the following from the telerik support staff.
"We are aware of this issue and our developers will ensure a proper fix for it in the next official release of RadControls - Q2 2012. Meantime, you can set EnableSingleInputRendering property of DateInput to false as a possible workaround."