Never in the history of modern politics have the people had so much access in central locations to legitimate polling for campaigns at the national level. I, myself, have darted between Nate Silver's www.fivethirtyeight.com and Chicago-based Real Clear Politics.
Nate monitors things in a scientific manner, meticulously studying what methodologies are used by each polling source then narrowing his choice of pollsters down to 8 to 10 reliable sources. From there, Nate further crunches the numbers and finds a logical baseline for each candidate in each contest and determines a pretty solid number. Real Clear Politics does not mess with the science of polling and the study of statistics: the numbers are posted and the raw averages are given.
RCP is like the guy who goes with you to the track and depends on the articles Racing Forum to tell him who is going to win the big race of the day. Nate and 538 are like the buddies who actually can read the speed charts from previous races, can tell you what the Beyer Rating means, and actually know if there is scientific validity to the Bounce Effect.
Nate and the guys in the lab have broken down each state and is just about the only major source showing that Barack Obama is above 270 electoral votes in a scientific analysis of the state by state polling (394 to be exact as of today). RCP and the major networks, including NBC, have topped out at 264 electoral votes.
Why? Florida, Election Night, 2000. Looking like a fool is no longer the business of major networks and especially in the calling of elections.
Truth be told, Obama is beyond the margin of error in enough states to take him over 270 electoral votes. Nate Silver doesn't have to worry about Steve Schmidt or Rick Davis calling him out in a conference call for making such a commitment. NBC and David Gregory do have to worry about being called out by staff of either side of the fence for making a 270+ commitment on the electoral map.
I would not expect any network to make a 270+ commitment until the last weekend of the campaign. Why? The last campaign conference calls will be on Friday, October 31 and the noise and confusion of the final weekend will drown out any bellyaching from the campaign at the short end of the stick on that call.