We are not aware of such a problem with RadToolTip and also our online demos work under IE7.
What I can assume that might be causing such an issue is that you have some specific code which causes a js error under IE7 only and this prevents the tooltip. However, I cannot confirm that this is correct and to locate the problematic code until I examine the setup. this being said, in case you cannot locate the problem, please send me either a live url or sample, fully runnable code along with detailed reproduction steps and I will do my bets to help.
the Telerik team