Why would the president authorize warrantless wiretaps of U.S. citizens is a timely question? The conventional press (and many politicians) are reacting as if the president is thumbing his nose at the courts and congress. Maybe the situation makes a warrant in advance impossible, as this Ars Technica article suggests.
It is a fact that the U.S. has captured laptops and cell phones from terrorists. Let's hypothesize that JohnDoe@yahoo.com is one of the captured e-mail contacts. Nobody knows who he (or she) is nor where in the world they are. It seems to me that the gist of what is happening is that the NSA puts a flag on JohnDoe@yahoo.com and waits and watches. The e-mail may be retrieved from anywhere in the world. Ditto on cell phone calls. The monitoring starts when the contact picks up the phone or retrieves the e-mail. At that point, it's too late to run down a judge.
In a separate Ars Technica article, the author suggests that the technology to do voice matching in real time on a large portion of the U.S. telephone traffic is not only technologically possible with today's computer power but is likely in place. Is hunting for the bad guy's voice a technology needle in a haystack? I suspect the answer is no, it's not impossible at all.