One needs to follow the money.
Russia has huge reserves of oil and natural gas, much of it just in the development stages and a lot of the developed sources in old, uneconomic fields. For Russian financial development and national prosperity, he needs the price of oil to remain at elevated levels to assure a healthy cash flow and the strong development. The demise of Saddam and the unleashing of the huge reserves of Iraqi oil into the marketplace would plummet oil prices, which in turn would further hurt the Russian economy - which is fragile even as it is.
IMHO, Putin is jockeying for some sort of American guarantees the bottom won't fall out of the price of oil next year in exchange for his support in overthrowing Saddam and allowing that extra Iraqi oil into the market. When he gets it, he will gladly come along. He is ideologically in our camp and in a murderous war with Al Queda as it is, but must look out for Russia's commercial interests at the same time.