That's what political history tells you. It is very difficult for a Democratic candidate to get to 50% in a Presidential elections. Since 1964, it has happened only once, in 1976, and that just barely. But they get to 48% quite regularly. Kerry got to 48, Gore got to 48, Clinton got to 49.2 in 1996. But never higher than that. So, that 48% number is a floor -- if Obama isn't polling even that high in the TIPP poll (which doesn't push undecided voters to "lean" to a candidate), he's in trouble. That's not to say that he is out of the woods even if he does reach 48% -- I'm getting to that later.
Now, compare GOP history. Since 1972, Republican candidates have often exceeded 50% (even in 1992-1996 if you count Perot as a right of center candidate, which he was), and they never did worse than 48 (Ford in 1976, Bush in 2000 -- again counting Perot as a right of center candidate).
This is why I said Obama wasn't out of the woods even if he gets to 48. Because McCain is also going to get to 48, and the history (including the most recent history we have, the 2004 election, a high turnout election) tells you that the remaining voters will break more Republican than Democratic. Even BIll Clinton, arguably the most popular Democratic President since JFK, never got past the 49.2 point barrier. Dole and Perot combined for 49.1 points -- a virtual tie. This in an election year like this one, where the media treated the Democratic candidate's victory as a foregone conclusion. If Bill Clinton couldn't get much past 49% even with an approval rating much higher than that, well, by now you should see the problem. And he was an incumbent President running for re-election, with the same adoring media that Obama has now, running against a much weaker Republican ticket that was down double-digits in virtually every published poll.
One additional reason why even 48 may not be enough: Obama is going to win by huge margins in places like California, New York, and Illinois. These wins are going to inflate his national numbers, but not help him win remaining states he needs to get to 270 EVs.
Just for fun, I figured out what the 2004 result would have been if you had excluded these states from the national totals. Here is what I came up with:
Bush: 53.2
Kerry: 46.8
And it gets worse. Most of the remaining big states are non-competitive blue states such as Michigan, New Jersey, and Massachusetts. Not only that, but he should win Illinois by about 6 points more than Kerry did (based on recent polling). By contrast, the only big non-competitive red state is Texas, and even Texas won't vote for McCain by the same 23 point margin of four years ago. A much more likely number is about 10 points, which is the margin that most statewide GOP candidates typically win by here in Texas.
Those who do not know their history are likely to be surprised and shocked by it. Or something like that. Speaking for myself, I'm bracing for the coming riots and screams of "racism, racism!" :)
You can add DC to your inflated list. Regardless of the candidate, it's an instant 200,000 (give or take) vote advantage for the Democrat.