I took it as a way for McCain to suggest Obama = Socialism without actually having to do it himself, and to paint Joe the Plumber as the embodyment of suspicious undecideds everywhere.
After the media did a little digging that the McCain campaign would've been wise to do before the debate, we learn that Joe "The Plumber" Wurzelbacher may be related to Robert Wurzelbacher of Cincinnati, Ohio, who happens to be Charles Keating's son-in-law. Robert Wurzelbacher was implicated in the Keating 5 scandal (just like McCain), and sentenced to 40 months in prison in 1993. Wurzelbacher is also a huge Republican donor. If that's true, I'd say ol' Joe isn't an impartial judge of Obama's policies.
Further, Joe hates paying taxes SO MUCH that the state of Ohio has a tax lien against him for $1,182.98 from January '07. He also has a credit judgment against him from a hospital in the amount of $1,261.
There were even rumors that Joe wasn't registered to vote, but it turns out his last name is spelled wrong on his registration. It's legal to have that error in Ohio but it's the kind of mismatch that would get him purged from the rolls if certain politicians had their way with "voter fraud" measures.