I suppose one could call oneself a "student of physics" while not believing in gravity, but I don't know how much progress s/he would make.
If you take gravity out of the study of physics a lot of the rest of physics falls apart.
It is the same with kamma and rebirth with regards to Buddhism, as far as I can see. Pull out a few bricks you don't like and the whole wall falls apart.
Regarding taking refuge in the Dhamma -- I don't know how one could truly take refuge in something that one believed incomplete or erroneous in certain places. I'm not trying to be judgmental, and as Cooran noted it's surely okay to be agnostic on some things and put them aside for the moment -- but I can't imagine really, truly taking refuge in any set of teachings if one rejects some of those teachings. What would be the point? It might be a bit like calling myself a Christian because I like a lot of the brotherly love teachings but I don't quite believe in the whole virgin birth and resurrection crap.
"To reach beyond fear and danger we must sharpen and widen our vision. We have to pierce through the deceptions that lull us into a comfortable complacency, to take a straight look down into the depths of our existence, without turning away uneasily or running after distractions." -- Bhikkhu Bodhi
"No man ever steps in the same river twice, for it's not the same river and he's not the same man." -- Heraclitus