I have already answered to your support ticket, so I'll answer this forum thread as well for the rest people in the forum.
Since Q1 2013 RadButton is rendered as an input wrapped inside of a span element. This change was essential in order for the RadButton to pass the Html5 validation. However, this leads to the following issues:
//Getting the button's wrapper
button1 = $find(
buttonWrapper = $telerik.$(button1.get_element());
//Set tabindex attribute to the span element
The priority of both issues have been raised, so that if everything is fine they might be fixed for the Q1 2013 SP1.
Please accept our apologies for the caused inconvenience
the Telerik team
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