One of my concerns about a potential Hillary-Obama unity ticket is that HIllary would not only not help Obama attract working-class whites, she'd actually worsen his problems with that group.
My logic starts with the fact that working-class whites who vote in Democratic primaries are often very different from the working-class whites who don't. In particular, Hillary may be as disliked by the latter as she is beloved by the former. (Matt Yglesias made a similar point last month, though I can't find the link to his item.)
Now, there is clearly a subset of working-class whites who aren't high on Obama either. (Race may be a factor, as my colleague John Judis writes this week.) The problem is that the working-class whites who don't like Obama may be different from the ones who don't like Hillary, in which case you risk alienating two groups of working-class whites by putting her on the ticket.
I mention this now because there seems to be some support for it in the latest Washington Post-ABC poll. According to the WaPo write-up, Clinton and Obama do more or less equally well among working-class whites in a matchup with John McCain: "Against Obama, McCain is ahead among whites without college degrees by 52 percent to 40 percent, not that different from McCain's advantage over Clinton in this new poll."
If you believe, as the CW has it, that Hillary wins certain working-class whites whom Obama would lose to McCain, then the math says Obama must be winning certain working-class whites that Hillary would lose to McCain.
My guess is that Hillary may or may not help you with the former, but she'd almost certainly hurt you with the latter.