I read more details at Slate on how some of the NSA's warrantless searches are inherently not the kind of thing for which you can get a warrant. Essentially, they are conducting traffic analysis (e.g. looking for certain patterns of, say, length and frequency of phone calls) on all Americans--at least all who communicate with foreign people or websites. This is in addition to the more targeted eavesdropping of electronic communications between Americans and individuals with known links to Al Qaeda. (Why such "known" links weren't used to get FISA warrents is a separate issue.) Traffic analysis is arguably a great tool for producing probable cause for conducting eavesdropping on particular individuals, but the traffic analysis itself is far too broad to have any justifying probable cause or reasonable suspicion.

I don't know enough about FISA to say whether or not the act has are loopholes big enough to let through all this surveillance. On the other hand, I've no trouble throwing in my two cents on its compatibility with the 4th Amendment. First, I don't accept the "border search" exception to the warrant and reasonableness requirements even in the case of the targeted NSA eavesdropping. Searching for contraband at the border is very different from reading the letters carried by a courier trying to cross the border, which is the low-tech analog of what the NSA is doing. It's not a reasonable search without additional information, the kind of information that is good enough for a warrant. As for the broader NSA traffic analysis, the "border search" exception makes even less sense.

I also don't believe the President has any "inherent authority" from Article II or anywhere else to conduct these warrantless searches. On this point, I shall link to a post of Orin Kerr rather than repeat his arguments.

I only see one way for all this surveillance to be constitutional: all the access given to the NSA by telecommunications companies is truly voluntary. If that's the case, then expect the Supreme Court to ultimately decide whether FISA has been violated. I expect this because although Bush might get Congress to retroactively legalize warrantless eavesdropping on communications involving essentially anyone the government asserts is linked with terrorism (see this poll), I strongly doubt Congress will acquiesce to the NSA's broader, TIA-style data mining of domestic communications. Yet, I don't expect Congress to muster a veto-proof supermajority to close all the alleged loopholes of FISA. Enter: the judicial branch.