The Russian military are promising again to deploy multiple warheads on Topol-M missiles - this time "in two-three years", according to the commander of the Rocket Forces. Well, it won't happen in two years - the START Treaty, which effectively prohibits MIRVing Topol-M, will not expire until December 2009. It probably won't happen in three years either - even if Russia will get serious about MIRVing, it would take time to test the new warhead and deploy it. In fact, I hope it won't happen at all, since there is absolutely no reason to make Topol-M carry multiple warheads. Other than, of course, to artificially inflate the total number of warheads in Russia's arsenal, so Russia won't lose in the meaningless "who has the most nuclear warheads" contest. As if it matters.
A lot of people in Russia, however, believe that the number of nuclear warheads is important, so we regularly see reports that declare that Topol-M will have seven warheads or that Bulava will carry ten. Nothing is, of course, entirely impossible, but it would be interesting to see what the actual MIRVing potential of these missiles may look like.
According to the START data exchange, Topol-M is a single-warhead missile with throw-weight of 1200 kg, Bulava is a six-warhead missile, whose throw-weight is declared as 1150 kg at the moment (START Treaty allows adjustment of the declared throw-weight based on the results of flight tests, so this number may change, but it is unlikely that it would change dramatically).
Historical data show that the weight of warheads comes to about half of the declared throw-weight of a missile. For example, this is true for a single-warhead Topol, whose warhead is under 500 kg, and for RT-23UTTH (SS-24) - its ten warheads weighed about 2000 kg (declared throw-weight of these missiles is 1000 and 4050 kg respectively). Another half of the payload is probably taken by the bus (for MIRVed missiles), missile defense penetration aids and things like that. There is certainly some room for maneuver there, but we can probably assume that this relationship will hold for a notional MIRVed Topol-M and for Bulava.
This means that each of the six declared Bulava warheads would weigh about 90 kg. The most lightweight warheads deployed in the Soviet Union and Russia so far were those of R-29R and R-39 missiles, with weights in the 110-130 kg range (this includes reentry vehicle body and electronics) and yields of 50 and 75 kt respectively. The R-29R warheads are unlikely candidates - they are fairly old. The R-39 ones seem to be too heavy to have six of them fit on Bulava - 75-kt warheads would eat up about 70% of Bulava's throw-weight instead of usual 50%. It is hard to tell without knowing the details of the missile design if this is going to be a problem.
Another possibility for Bulava is to have a new warhead that would resemble the U.S. W76, deployed on Trident I C-4 missiles. According to Soviet data, W76 has the weight of 91.7 kg (of which 61.5 kg was the nuclear charge, 22.7 kg - reentry vehicle body, and 6.7 kg - electronics). With the yield of 100 kt, it had a yield-to-weight ratio which is slightly better but comparable to that of the R-39 warhead (100 kt/61.5 kg vs. 75 kt/about 50-55 kg, which is about 20% difference), indicating that development of a 90-kg warhead with a 75 to 100 kt yield would not require any breakthroughs and could probably be done without nuclear tests.
As for Topol-M, it is possible that it could carry the same 90-kg warhead, should one be developed for Bulava. In this case, Topol-M would indeed be able to carry seven of them, although it would be somewhat unusual for a land-based missile to have small-yield warheads. Another option for Topol-M would be to have three warheads of the type deployed on R-23UTTH/SS-24 - at about 200 kg each they would take about half of the throw-weight of the missile. With the yield of 400 kt, they would be more in line with the historic trend.
But then again, nothing of this really matters - none of these new warheads have any reasonable mission. The "MIRVed Topol-M" and, to large extent, Bulava are political projects, designed for the cold-war style competition. They may as well carry warhead mockups filled with concrete - the effect would be pretty much the same.