The letter stated that the US would refrain from building a missile defense system in the Czech Republic and Poland (to which the Russians are strongly opposed) if Russia would help stop Iran from developing long-range weapons.
Russia has a relatively close relationship with Iran and, according to Baker, could exert influence on Iranian decision-making.
Sergei Lavrov (Russia's foreign minister) and Hillary Clinton are meeting this Friday in Geneva, and it's expected they will discuss this issue. Medvedev has not yet responded to Obama's offer. The Administration's position:
" 'If through strong diplomacy with Russia and our other partners we can reduce or eliminate that threat, it obviously shapes the way at which we look at missile defense,' Under Secretary of State William J. Burns said about the Iranian threat in an interview with the Russian news agency Interfax while in Moscow last month delivering Mr. Obama’s letter."This is another example of good, proactive diplomacy by Obama and his team. He is putting the ball in the Russians' court - as in negotiating a contract, I assume that this is a good diplomatic tactic.