Bruce Schneier writes, that the Associated Press says:

In the last few years, Caller ID spoofing has become much easier. … For instance, sells a virtual “calling card” for $10 that provides 60 minutes of talk time. The user dials a toll-free number, then keys in the destination number and the Caller ID number to display.

Basically, fraud can be committed but you cannot protect yourself from others finding out your number. T-Mobile switched their voicemail system last year to require a PIN number to access it, because people were using called ID spoofing to “hack” into other peoples’ voicemail.  Unfortunately, other carriers like Cingular still rely on things such as called ID.

James Lick posts a comment saying:

There are two ways that the recipient of a call can know the calling number, Caller ID and ANI (Automatic Number Identification). Caller ID Blocking DOES NOT block ANI.

If you are using Caller ID Blocking to make sure that telemarketers don’t get your number, you’re just fooling yourself. Of course, Caller ID Spoofing will also usually spoof ANI as well.