Hmm well, those on the left will be a little worried he'll rob them of their entitlement at the Presidency, like they think he did in 2000 - but well, I'm not fussed. Frankly, he might help Obama as claims of Obama being on the left will be diluted by Nader standing, as if one can point at Nader and say "hey he doesn't think I'm socialist enough" (or "progressive" as statist thieving is called").
Funnily though, Ralph Nader said to NBC according to the Sunday Telegraph "if the Democrats could not win this year by a “landslide” despite his involvement, then “they should just close down". He's probably right.
I despise Nader, but he does have principle - he at least gives those who want more government, more taxes, more regulation, destruction of private property rights, surrendering US allies to their enemies and hatred of business, someone to vote for. Who do those who want less government, less tax, more freedom, wider application of private property rights and respect for wealth creation, along with standing with US allies against attack and oppression?