Among others, Nate Silver has been putting a Romney-Obama match-up at nearly even odds. This surprises me, but apparently not InTrade, which as of this writing has a similar valuation (51.7% in favor of Obama). Then again Romney tends to poll the same right now as a generic Republican, and maybe as people differentiate him from that, these numbers will change.
The upcoming election makes it interesting to look at incumbents and changes of the guard. Since the start of the twentieth century, there have been 19 elections where an incumbent ran. Of those, the incumbent won 14 times (74%). Of the 5 that lost, 3 had clearly extraordinary circumstances. Taft lost when he faced not only the Democrat Wilson but also TR's Bull Moose party, which split off a large chunk of the Republican vote; Hoover lost after the Depression began; Ford lost after Watergate. (The remaining two were Carter and Bush 1.)
For presidents of >1 term it's different. Since 1900, if you've been in more than 1 term (which counts partial terms), 2-to-1 at the end of your last eligible term, you're replaced by someone from the other party. This happened to Wilson, Truman, Eisenhower, Johnson, Clinton, and Bush 2; whereas for >1 term presidents replaced by someone from the same party (3 presidents), all 3 were Republicans, and 2 of 3 VPs succeeding presidents - TR to Taft and Reagan to Bush 1. Hoover was the third, and was not Coolidge's VP.