Our trips have been in 2001, 2004, 2009, 2011, and one coming up in June 2013. I've gotten several PIN codes every year since somewhere around 2003 or 2004 -- some by e-mail, some by snail mail. I don't remember if we used a PIN in 2004 or just the regular discount. In 2009 we used the Buy 4 Get 3 Free deal. In 2011 we used a Free Dining PIN. For our upcoming trip, we're booked with the Visa (general public) discount.
On Wednesday of this week, I got a PIN code via snail mail. It was very vague -- "Save at least $125 per package at Value Resorts.... Save at least $250 at Moderate Resorts.... $550 at Deluxe and $650 at Deluxe Villas...." I sent the info to our TA and she called Disney to check on the specifics of the PIN. It turned out to be the same discount as the current general public discount.

For those who were wondering if Disney sends PINS to regular/frequent visitors, this particular postcard reads, "You're a regular here at Walt Disney World Resort! We want to give you a chance to enjoy 5 nights and 6 days...."
That being said, in 2011, my sister who hadn't been to Disney in more than 15 years called to make a reservation and the agent said, "Oh, it's you're lucky day! You have a PIN for Free Dining attached to your name!" She was most definitely *not* a regular visitor.
Also, for those of you wondering what the e-mails look like: they always come from Disney Destinations (the subject line varies). Somewhere in the body of the e-mail it will state something about "be sure to mention PIN Code XXXXXXXXXXXX when making your reservation." I've gotten in the habit of always checking my spam folder (I sort it alphabetically by sender to make it easy) because that's where these e-mails always seem to end up even though I mark them as "safe" every single time.