Josh has linked to a variety of tech/sigint/spook sites mulling the interface of technology, tradecraft, and gumshoe lore seeking an explanation for the strange decision by the administration to eschew the legal route to sigint for a darker and more dangerous course.

Keeping in mind the references to "nimbleness", "soft subtle triggers", "roving",etc, herewith a theory by way of scenario:

Habib gets a call from Karachi on his home phone; or cell phone or at the corner pay phone by prearrangement.

Knowing that calls originating from abroad are radioactive (indeed, in our scenario the originating pone number is being watched routinely) he tells his caller, "hang up--I'll call you back from a (different) payphone.

At this point the only fix on Habib is his voice, and his geographical locus.

The NSA now turns Echelon loose on the forty or fifty exchanges that serve the twenty miles around Habib's last known location, and they sweep everything into a hopper, where they screen for Habib's voice. Ditto for anyone (foreign or domestic) who Habib calls from now on.

This fits the model of a program which would not stand FISA scrutiny, and which if presented for congressional debate might screw the pooch insofar as alerting otherwise hip but in this case clueless terrorists in re:voice recognition technological advances.