I can't quite pin down everything that makes me suspect him, but it sure seems like he'd be in the PERFECT position for manipulating the survivors. So far it seems like the Cylons go in more for manipulating/influencing than necessarily being in charge.
After all, he supplied the drugs to the President that caused her hallucinations, he (possibly conveniently?) wasn't on Galactica when Sharon shot Adama,... and I can't help but wonder if perhaps Roslin's cancer isn't spreading or is perhaps treatable. But by telling her it has spread or can't be treated, you're certainly influencing her actions.
I dunno. Maybe it's all nothing, but it sure seems convenient they just happened to have ONE doctor survive with them.