There have been a lot of misuses of the term socialist by the McCain campaign with regard to Obama. I think true socialists are appalled that people are giving the ultimately capitalist underpinnings of Obama's economic package such a lofty and advanced title!
Let's take the bail-out of Wall-Street as an example.
True free-market capitalists are perhaps uneasy about the GOVERNMENT intervention into the capitalist system that Wall Street represents.
Socialists are uneasy about the government intervention into the CAPITALIST system that Wall Street represents. The Bail-out, in the socialist view, ultimately reaffirms the capitalist system rather than truly developing a socialist system.
It is one thing to distinctly disagree with your opponent's economic policies--that is part and parcel of Presidential debate and politics--but it does not excuse fear-mongering by mislabeling (this particular label, I don't particularly see as a negative one, but a great deal of Americans do).
By the way, Ken Schenck has a very nice, thorough post on the philosophical and historical developments of modern economic theory here.