I've been wondering--why use the NSA in particular to do the spying on Americans? It would be the perfect agency to use to spy on as many people as possible.....
(more on the flip)
In general, the NSA doesnt go tapping individual phones; rather they have links into the telecom networks, siphon off as much of world telecom traffic (both text and voice) as possible, run it through massive computer banks for smart keyword searches, and then flag suspicious items for analysis by specialists. (I know this sounds tinfoil-hattish, but it is well-attested. Check out ECHELON entry in wikipedia and the links from there)
Anyway, normally it is illegal for the NSA to analyze US communications in this way. If Bush has been using the NSA to spy on Americans, then it probably isnt an issue of tapping phone lines of suspects, but rather of allowing the NSA computers to analyze all of our US telecommunications. This would at least be one plausible explanation for the obvious question of why Bush wouldnt go to the special intelligence courts for warrants--because the program revealed was not directed at individuals, but directed at certain keywords which could have appeared in anyone's conversation by chance. There is no way to get a warrant for the whole country.
Another point in favor of this possibility is the fact that the program has been re-authorized every 45 days. In other words, it is a regular, ongoing exemption from spy rules, not a pattern of particular exemptions authorized on a case-by-case basis by the president.
If this is the case, then I think it probably puts impeachment into play as a realistic political possibility. Conservatives are probably are too self-absorbed to care if the president breaks the law in torturing foreigners, but if he has broken the law to spy on them personally, then people will get very pissed very quickly.