Here is an interesting tidbit of law.  The US Congress would like to make it illegal to spoof Caller ID.

Title: A bill to amend the Communications Act of 1934 to prohibit manipulation of caller identification information.

Sponsor: Sen Nelson, Bill [FL] (introduced 2/28/2007)  Cosponsors (4)

Latest Major Action: 6/27/2007 Senate committee/subcommittee actions. Status: Committee on Commerce, Science, and Transportation. Ordered to be reported with an amendment in the nature of a substitute favorably.

You can find more details here.