Due to the increased number of developers trying to use the RadToolTip in DNN environment, we made extra effort to modify [some of] the MS AJAX code so that it returns correct values in quirksmode (non-XHTML mode).
The Q1 SP2 will be out next week, and our suggestion is to wait until then, and give the tooltip a try - in a non-XHTML page.
On a side note, the tooltip does work fine in XHTML mode, so changing the doctype properly
should have delivered the right results. If you open a support ticket and send us a client-side page dump, we will look into it and see what the problem is.
the Telerik team