I know this used to work, now (Pre3) for some reason it doesn't:
Select item in grid, click on associated date field shown in properties pane, attempt to delete the value set for the date field. Whenever I press enter, the value reverts to the original value, I can't set it to nought through the properties pane...

If I delete the item using "Remove from this grid only" command, I can achieve what I want.

Thanks