And have you thought about taking a week off altogether? I know it sucks to do so, but at this point, I'm not so sure just easing back is doing the trick.

I'm not overly familar with hamstring issues. Mine get tight, but that's about it. I do know that my own tendency to ride or run through pain generally just leads to more and/or prolonged pain. In the very least, stop doing any climbs in spin class.

I agree with Jonathan that clipless pedals will help you better engage your hamstrings, but that alone won't cause an injury. Temporary soreness, yes, but probably not injury. Did this also coincide with him raising your saddle? That could also be a culprit I suppose in that the higher the saddle, the more you're stretching your hamstring.

To me, all of this just reinforces the fact that I think you could stand to add some yoga or pilates into the mix. Cycling involves such a limited and repetitive range of motion. It helps to do something to counter the long hours on the bike. In reading your 2011 cycling goals thread, I see that your winter workouts largely revolve aroung cycling or spinning. I, personally, use the winter to broaden things a bit and give my body a chance to move in different ways. I find that I'm less injury prone when I do that.
Just my random thoughts....FWIW.