
It also goes like this:

FED1: We need to listen into <mafia head/druglord/terrorist/BBS sysop>'s
      conversations but don't have probable cause for a tap.

FED2: Who do we have a tap on?

FED1: <some other enemy of the state>.

FED2: So get the keys - we can always claim that phone's being used on the
      tapped line.

There's got to be some way to associate an instrument with an individual or
the state effectively has a tap, possibly not admissible (given the War on
Drugs exceptions to the 4/5 ammendments, who knows) but none the less
listenable tap on any phone.