[eluser]champs[/eluser]
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.