Millions of people are looking forward to November 24th--US carriers won't be able to lock your phone number. It hasn't come freely though... You've been paying for it since Q2 of this year. It varies by carrier, but it is usually around $1 a month.
CNN points out that even though it doesn't sound like much, it will mean tens of millions of dollars extra per quarter. For example, Verizon Wireless has 34 million customers. The costs for Verizon are about 10-15 cents a subscriber per month. Not a bad profit margin.
Also pointed out is that some carriers are charging a lot more than others. Nextel has been collecting a fee of $1.75 a month since April, while Cingular has only been charging 32¢ to $1.25 (depending on the state).