>What is the rule of thumb for when to use a NODEFAULT? I find myself using it on a trial & error basis. Is there logic to it's use?

It's pretty confusing, because it's different for different controls and different events. The difficulty comes in determing what native behavior can be suppressed with NODEFAULT. There are a few key methods to remember that you can use NODEFAULT. I won't be able to give you a complete list, but the ones that come to mind are:
Grid.BeforeRowColChange (prevents the focus moving from one cell to another)
Control.KeyPress (prevents that controls from processing the keystroke)
Form.QueryUnload (prevents the form closing when the user clicks the 'x')

>
>Also what can you tell me about programmatically opening a combobox dropdown on entry?
>I didn't get the thing to work with a downclick in the setfocus.
>

put:

KEYBOARD '{ALT DNARROW}'

in the control's gotfocus event.