Sometimes my feet go numb when I'm spinning, and I've found it means that I'm tensing my leg muscles. When I relax (which improves your effort, BTW) the numbness will go away. Also, another mistake I made when I started spinning and road riding w/ cleats is that I had my shoe straps a little too tight. They felt comfortably snug to me - but the feet swell a little during a ride/spin, and if the shoe's too tight, numbness can result.

So many little details to hammer out, aren't there?