Hi Dan,
I don't believe there is a built-in way to accomplish this but you could do so by creating a user event mapped to pbm_dwnmousemove and then change the tool tip on the fly depending upon where the xpos is for horizontally positioned radio buttons and the ypos for vertically positioned radio buttons. You would also want to check for the mouse position being over the object in question and if it isn't do nothing.
The code in the event would look something like the following for two radio buttons side-by-side:
If dwo.Name = "which_continent" Then
If PixelsToUnits(xpos, XPixelsToUnits!) > 950 Then
dwo.tooltip.tip = "Select this only for North Amercia"
Else
dwo.tooltip.tip = "Select this for all other continents"
End If
End If
The first tooltip setting is for the radio button on the right and the 950 (PBUs) represents the mouse position just before that radio button starts.
Obviously a downside from coding it as I have is that every time you modify this object (EG its position, the number of options/ radio buttons etc.) you will have to manually change the above code. However, depending on your circumstances and use case it may work for you.
HTH...regards,
Mark