help - how to answer this most common interview question?

I got laid off this week and let go in 3 months (end of oct).
so i am looking for new job now...i am always not quite sure on how to answer this type of question from the employer :" why would you leave?"

normally i would say somethiing like :" i am looking for new challenge"
---that makes them think "why? your resume looks like you have accomplished so much challenge for your current position" ---so to me , this reason is not very strong.

my question is in my current situation -- should i mention lay off at all during interivews within these 3 months or not?
how about after 3 months...must i mention that because i will be out of work any way then?

what is your opinion on that?

thanks!!!!!