There are 19 states plus Washington DC that John Kerry won in 2004, when Bush beat him by 286-251 in the electoral college. To win, Hillary needs to add Ohio or Florida. There are three other fairly closely fought 2004 states, which the Democrats ought also to win this time: New Mexico, Iowa and Colorado (adding two of them helps Obama in what follows, so we are being fair). Let's look at the states that Clinton and Obama have won, and focus on those 24 states plus Washington DC (as people in Florida and Michigan will vote, their college votes are included).
Based on the electoral college for each of these states, Hillary has won 202 Electoral College votes to date, and Obama a mere 97 such votes. There are two of these states still to be contested - Pennsylvania, with 21 electoral college votes, where Hillary seems set to win, and Oregon, with seven votes. Even if we exclude Florida's 27 college votes and Michigan's 17, Hillary still beats Obama 158-97, or 179-104 if she takes Pennsylvania and he wins Oregon. So, if we're talking electability in the states that matter, then Hillary wins hands down. And that's also why the big state primaries are rather more important than the folksy caucuses or no-hope Democratic states where Obama has been doing especially well.
UPDATE: Michael Savage at the Independent Open House has been thinking along the same lines.