In addition to Travis' great scenario, I give you other, (less plausible?) alternatives:
Since there is enough already out there, allow me to speculate on the email account password sharing.
The bad guys have a way to defeat carnivore. The way they do this is by logging into an internet based email account (yahoo email) and composing a message - with no send to address.
save the message and log-off (no send executed)
The message sits on the yahoo server.
The next bad guy logs on to that very email account (note the passing around of Berg's password) reads the message then deletes it.
Carnivore never gets to intercept the email - because it is never sent.
Berg was either used by the bad guys, for his email account (probable), or he was an intelligence asset and this was a setup to access Moussaui's communication network (less than zero probability).
If the later, this guy was a hero - and a new star will go on the wall - with no name attached.
If the former, this guy was today's version Forrest Gump.
Regards,
Bump!
124 covers a lot of the spectrum of possibility.