Even though I feel that I tend to keep up well with things related to Identity and Identity Theft, there are always things that slip through the cracks (some, even after I have gotten wind of them).
In the case of opting out of all of those pre-screened credit card offers, I vaguely remember a friend sending me an email about it a year or two ago, but that never made it above the line to actually have me take a look at it.
However, yesterday, when I was logging into Paypal to pay for an eBay auction, Paypal told me I was pre-qualified for one of their credit cards and I noted that they had some stuff about opting out, so I started digging into it. At first I thought it was a way to opt out of the stupid offers they have following the login, but was disappointed to find out it was just some info about opting out of pre-screened credit card offers.
Since I get something like 10 to 20 of these every week, getting rid of them would still be a good thing, so I dug further including a visit to the Federal Trade Commission's web site and some poking around in other sites to verify the authenticity of what I had read.
You can opt-out for 5 years or permanently (depending upon whether you want to just fill in a form or also mail it in). The form does require all the information that one would need to steal your identity (name/address/DOB/SSAN, etc.) and so it did give me pause and caused me to do some secondary research, but everything I've found at a number of reputable sources convinced me they're on the up and up.
Update: As Pat pointed out in his comment, you don't have to enter the DOB/SSAN information. I'm not sure if this impacts how well the opt-out matches with the records in each system. I supplied it all as they have it all already.
If you're cautious about entering the data online (I wasn't), you can call toll-free 1-888-5-OPTOUT (1-888-567-8688).
I recommend that everyone opt out permanently (unless you like the idea of getting identity theft invitations in your mailbox every day).