I tried to replicate the issue which you described, but to no avail. Attached to this message, you will find the code which I used for testing. Please, take a look at it and let me know if there are any differences at your end, which I may be leaving out.
Note that failed Viewstate load could be caused by dynamically adding controls in Page_Load
method. It is more likely that server-side code causes the issue and not the declaration of the controls.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed