I agree, the statement that Tsvetie used can be interpreted in this way. However, I would like to stress out that the RadToolTip "Prometheus" control can be used with RadGrid with Ajax enabled. The only difference is that the implementation for now is not so straightforward as with other controls.
I believe the information below will shed some light over the situation:
When you enable the Ajax of the RadGrid, any postback (raised from within the RadGrid), would result in an AJAX request that would update the RadGrid control, and the RadGrid control only.
For the RadToolTipManager to "tooltipify" the RadGrid control after Ajax update, "something" must inform RadToolTipManager as the manager must go again through the controls with IDs from its TargetControls collection and tooltipify them.
By the way the same logic apply to cases when you have tooltipified an element that is in an UpdatePane, RadAjaxPanel, etc. In this case, to make the implementation easier for our customers, we added some code that would "listen" for ajax requests and check whether it needs to tooltipified any updated by ajax request controls.
Currently, as the Ajax request made by the RadGrid (with EnableAjax=true) differs slightly from the standard ajax requests, the RadToolTipManager cannot register these requests.
Of course we will do our best to improve the situation and to make the tooltipifying of RadGrid easier, however this will happen in a future version of the controls. This task is not easily achieved and we will need more time for investigation and implementation of the new logic. I hope that you understand our position in this matter.
This being said, for the time being we suggest to use the workaround that was shown in this forum thread. If you experience any problems with it, please send us a sample project in a support ticket (as you know, you cannot attach files in the forums) and we will check it right away.
the Telerik team