The best argument for Lance being clean is this: he has had his body ravaged by disease, used nasty drugs to get through it, is probabaly sick at the thought of more such drugs. After winning several Tours, he has millions of dollars stacked away; if he used drugs to get the $$$ he could retire now a rich man, and not endanger his health any further (especially if you buy the argument that taking such drugs led to the cancer in the first place).
Hard as it is to believe that a clean athlete could beat all of the freaks in the Tour who ARE on drugs, if anyone could do it, it is Lance.