Apparently, Russia cut off fuel to Iran's new Bushehr nuclear plant until they comply with the UN's several ultimatums to stop enriching uranium. From the NYT:
Recently, however, Moscow and Tehran have been engaged in a public argument about whether Iran has paid its bills, which may explain Russia’s apparent shift. But the ultimatum may also reflect an increasing displeasure and frustration on Moscow’s part with Iran over its refusal to stop enriching uranium at its vast facility at Natanz.
“We’re not sure what mix of commercial and political motives are at play here,” one senior Bush administration official said in Washington. “But clearly the Russians and the Iranians are getting on each other’s nerves — and that’s not all bad.”
It might be a financial feud, but it could partly be a power struggle. Russia letting Iran know just who needs who. I don't think it's an example of Russia asserting it's comparative power advantage over the UN, because well, the UN has proven its complete lack of effectiveness.
One thing is certain, Russia did not suddenly decide to warm up to the West, they after all, built that power plant and expect a return on that and many other of their investments in Iran.