President Obama marks his 100th day in office tonight with his third primetime press conference since January. The President's request for air time adds a strain not only to each network's news resources, but also to their overall bottom line.

It's no secret that the 8pm hour generates much-needed ad dollars for broadcast TV networks. In February, ABC, CBS, FOX, and NBC combined to average $21.5 million in revenue on Wednesdays from 8pm to 9pm ET. With that kind of money in play, networks are forced to balance their public service duties with financial obligations.

So what happens to advertisers who pay good money to place ads on pre-empted primetime broadcasting?

"In a situation like that, networks will find other ways to make good on their deals with advertisers," says Annie Touliatos, VP for Sales Development at Monitor-Plus, Nielsen's advertising intelligence service. "They can shift programming or offer to run the ad another week. They can also spread the ad buy over several spots that offer the advertiser the same level of viewer impressions. The key is for advertisers to ensure they will reach their target audiences effectively."