Hmm. An easier method perhaps...

Change the Valid code for the texbox object which is inside of the grid. You can access the Valid code by finding the textbox, which is under GridColumn, in the Properties window. The double click on the Valid method below. Now you can check the value of what is in the box. If you return .F., the user will see a box that says "Invalid Input" or something like that, and it'll stick 'em back in the box.

HTH,
Adam