There are two important and impressive factors to note in this decision:
1) Obama has quickly and decisively set his mark on the Democratic Party. Traditionally, the Presidential Nominee becomes the de facto leader of the party, and remains so either until defeat or through the Presidency. That Obama has moved so quickly after seizing the nomination to make a significant change within the DNC shows that he is completely ready to take charge. On the other hand, he is not making change just for the sake of it - he has also indicated that he is happy for Dean to remain as Chairman, something that makes a lot of sense given the synergies between Dean's 50-State strategy and Obama's own plan to compete in a much broader swathe of the country.
2) With this move, partly a symbolic sacrifice (lobyists only contribute a very small part of the DNC's funding) and partly real pain (PACs are large givers to the party) Obama has made it clear that his expects the Party to reflect his core message - a new kind of politics, strong ethical standards and a move away from Washington insiders. He has also innoculated himself against accusations of hypocracy later on - since the DNC will provide major support to the campaign, if he had not imposed this rule Republicans could have claimed he was having it both ways.
This is an impressive start. But remember, without lobbyist and PAC donations to rely on that only leaves Barack with one remaining source of funds. You. Don't let him down.