One thing working against Obama is his percentage of people who don't know enough about him to rate him, 41%. By comparison, Giuliani's "unknowns" were only 9%.
If Obama can generate a lot of positive PR (which should not be too difficult with the liberal Media), he might even knock Hillary Clinton from her perch.
Below is a listing of potential candidates, with the "unknown" rating in parentheses (I am excluding other political figures who cannot or have no announced intentions to run, such as Bill Clinton, George Bush, and Nancy Pelosi):
1) Rudolph Giuliani - 64.2. (9)
2) Sen. Barack Obama 58.8 (41)
3) Sen. John McCain 57.7 (12)
7) NYC Mayor Michael Bloomberg - 51.1 (44)
8) John Edwards - 49.9 (20)
9) Sen. Hillary Clinton - 49 (1)
10) N.M. Gov. Bill Richardson - 47.7 (65)
11) Sen. Joseph Biden 47 (52)
13) Gov. Mitt Romney - 45.9 (64)
14) Former VP Al Gore - 44.9 (3)
16) Sen. Evan Bayh - 43.3 (75)
17) Newt Gingrich - 42 (15)
20) Sen. John Kerry - 39.6 (5)
Don't count McCain out. He certainly put in a good showing, even if he was only third.
As for Hillary, she is in an awful position. Unless she can change the way a lot of people already view her, she will lose the middle to any of the GOP candidates. Obama could possibly steal the Democrats from her.
If the 2008 presidential campaign were a horse race, I would say Giuliani vs. McCain is too close to call yet for the GOP nomination. For the Democratic nomination, I would say Obama has an early lead over Clinton, but we don't know how he will do on the long track of presidential politics.