Just got 09 mazda3--27,000M--on highway going 60-65--when I take foot off gas car bounces up and down like there are bumps in the road but the road is smooth--any ideas of what may be causing this?

 

That sounds quite odd, first thing that comes to mind is bad alignment, but that should vibrate more than bounce.

Is this an automatic?
How quickly is it 'bouncing' and how quick is the bounce, once per second etc...
Do you feel the car speeding up and slowing down at all while it's happening?
And you should be able to repeat the problem for the dealer and have them fix it.