datetime_select and preserving NULLs

I have several datetime columns and in my 'new' and 'edit' views sometimes I would like to set a specific date time while other times I would like to leave it null. For the life of me I can't figure out what I should be using in my _forms.rhtml so that datetime_select accepts a NULL value for the datetime instead of a preset value. I've tried setting the default value of the column to NULL or '0000-00-00 00:00:00' but to no avail.

I see there is an open defect on this issue: but was wondering if there was an easy work around that someone could recommend.

Thanks, Matt