Taken from the sample blog tutorial.

Does db->insert not kick off a NOW() function on DATETIME fieldtypes?  Seems to always leave the value NULL.

Is there a way to get it to fire off NOW(), or do I need to format a string to pass with it?

All strings passed to Active Record are inserted as strings, so NOW() is wrapped in quotes, just like everything else, and you will need a function that returns the date string you need.  In MySQL, you’ve got an ON UPDATE CURRENT_TIMESTAMP attribute (that you can set on ONE field, IIRC), but if you have multiple date fields, don’t bother.

And just to get out ahead of you, a special exception for NOW() means special exceptions for *every* pet function someone cares to use in their statements.