I can offer a few theories as to why Ralph Nader has announced that he will once again run for President.
The first, and that to which I subscribe, is that he takes some sort of perverse pleasure in functioning as a spoiler. Whatever were his intentions in 2000, he was a spoiler for Gore, which resulted in the election of George Walker Bush. Nader had less of a chance to spoil things for the Democrats in 2004, and certainly less of a chance to move the Democratic Party to the political left, but he could and did spoil things for the Green Party, destroying them by running against their candidate. In 2008, Nader doesn't have a good chance of spoiling things for the Democratic nominee, but it's better than were his chances in 2004; McCain is not as loathed by the left as was Bush.
An alternate theory would be that Nader is trying to give Obama some center cred, on the presumption that Obama will indeed be the nominee. Seeking first the nomination of the Democratic Party, Hillary Clinton and Obama have each positioned themselves significantly to the left of where the last successful Democratic Presidential nominee, William Jefferson Clinton, did when he was running. McCain is a war-hawk, and a conservative on some issues, but he is a centrist or even to the left of center on other issues. To win the election, the Democratic nominee will have to seem more centrist than presently does Obama or Hillary Clinton. On top of some rhetorical restyling by the candidate, it could help if Nader provided apparent contrast.
Of course, Nader is more likely to function as a spoiler if Hillary Clinton is the nominee, because she has at times been a war-hawk, and there is less expectation that she would withdraw troops aggressively from Iraq than that Obama would do this. So it is possible that Nader has announced when he has in order to further weaken the Clinton campaign. In that case, we have Nader acting as a spoiler of some sort for Clinton (and possibly for the Democratic Party), and positioning himself to make Obama look more centrist should he gain the nomination.