The question of when the government needs warrants to eavesdrop hasn't gotten the attention it deserves in part because it's so complicated and difficult to explain.
One of the experts I follow most closely on government spying is Julian Sanchez of the Cato Institute, who has a knack for explaining complicated subjects in comprehensible language. He has produced a video summarizing the Senate's recent debate on the FISA Amendments Act (which I wrote about here), including key video clips excerpting arguments of the legislators involved. If you've been wanting to know more about this undercovered story it's a useful resource. And it gets across how difficult it is to convey this material to a general audience.
Obfuscating is easy when everything is cloaked in bureaucratic jargon and official secrecy, which permits the government's defenders to mislead without any fear of being directly contradicted. My favorite part of the video is where Senator Dianne Feinstein offers, as if it would prove something, to waive a classified document in the air, only to comically fail at even that little transparency. If you're following the subject closely, don't miss this takedown of the Wall Street Journal's misleading commentary.