How to put a tooltip on a user-defined function

In Excel 2007, how do I add a description and parameter hints to a user-defined function? When I start typing a function invocation for a built-in function, Excel shows a description and parameter list--a tooltip. I'd like to do the same for the functions I define.

Not just for the formula insert wizard, but in the formula box, so if I key "=myFun(", at the "(" the tooltip pops up just like it does for "=average("

There's no help in VBA Help, none on MSDN and none on any of the Excel and VBA dedicated forums I can find, so this is clearly a long shot.