The first New START data exchange scheduled for March 22, 2011 should finally answer the question of how many warheads are deployed on the RS-24 version of the Topol-M missile. Some press reports mentioned four, but Russian press is notoriously unreliable. Commander of the Rocket Forces also said at some point that RS-24 will carry four warheads. But Russian generals are not much more reliable than the Russian press.
My analysis suggested that the number is three, but it is an estimate based on circumstantial evidence. Now we have Vladimir Popovkin also saying that "Topol-M [i.e. RS-24] can carry a maximum of three warheads" (via Russian Military Reform - a very interesting blog maintained by Dmitry Gorenburg). But then again, Russian generals are not particularly reliable. I guess we'll have to wait for the New START data.