I need to clarify that
The MAC address only carries as far as the next router node - and could be picked up there only
However theres nothing to stop a piece of software picking up a MAC address and transmitting it to a third party. Or for a bug or trojan to transmit that info.
Thats really the point I was trying to make, I just worded it badly
Of course what the network sees is the MAC address of the router - not of the user's PC. BUT, if the authorities can gain local access to the network and find the MAC address of the specific machine, then all bets are off
A very simplified example of how this can be used
How the FBI used computer MAC addresses against Lulzsec hackers | Hacker 10 - Security Tips
the next comment is a bit of a double post as I've added it to Probe's thread on a similar issue, but for someone not bothered about the authorities, who just wants to encrypt mail, take a look at these offerings
Hacker 10 - Security Tips | Email encryption