Rock459 and NightSwimmer have it exactly right. The word "socialist" has become infinitely malleable. As Lewis Carroll had Humpty Dumpty say to Alice: "When I use a word, it means just what I choose it to mean - neither more nor less."
To this day wing-nut conservatives regard FDR's New Deal programs like Social Security as being socialist. When George Bush and John McCain talk about "privatizing" Social Security they are continuing this time-honored GOP tradition.
So when conservatives call Barack Obama a "socialist" they could mean anything from being a defender of Social Security to being a crypto-Stalinist. That very ambiguity just makes it another cheap shot.