Don't book your tickets to Iranian vacation spots yet. Obama's cautiousness towards Iran is becoming apparent, and there is unlikely to be a diplomatic honeymoon between the US and Iran.
But this makes sense. With Iranian elections coming up, Obama has no intention of appearing to support president Ahmadinejad. If Obama were to respond with glee to Ahmadinejad's congratulation on victory, it would be a signal that Obama was fond of the Iranian president, and would probably help Ahmadinejad's chances in the June election.
Obama will have to toe a careful strategy of showing openness to negotiation and embassy in Iran, while still criticizing the hardline policies of its presidency, to convince Iranian moderates--hopeful for a good relationship with the West--that Ahmadinejad has to go in the June elections.