Well, Morton Halperin has written a very good summary of the many legal and Constitutional issues involved. It's not the shortest summary, but it is one of the best.
Halperin not only explains the role different legal standards play in determining if the President has the right to conduct warrant-less wiretaps, but explains the historical considerations and the intentions of Congress.
The intentions issue is one of importance simple due to the fact that the one of the legs of administration's argument rests on its interpretation of Congress' actions. If Congress specified a particular reason for including or excluding certain powers, and the President reads it another way, well…rhetoric aside it leaves little room for argument.
The President's argument rests on the administrations interpretation of the Constitution's description of a President's power, Article II (specifically the part on his role as Commander in Chief, section 2), and Congress' passage of the Authorization for Military force in September of 2001.
The case revolves around the issue of, "Why didn't the President go to the FISA courts?" The process was already in place to do exactly what the President said he needed, to act quickly and secretly. The only reason to avoid that process was to avoid having to approach the FISA courts; courts that haven't met a government request it did like.
Why avoid a system that was virtually a rubber stamp for warrants related to national security and provided complete secrecy?