Copied this from AnandTech forums, my year Honda is not included. I didn't click on the links to see what the story is.

Honda is giving free service for the vehicles listed below. Give your dealer or the # on the letter a call with your VIN number to
see if it is qualified. Thanks LordRamZ.

* 1995 models: Honda Accord V6, Acura NSX, Acura 2.5TL
* 1996 models: All models
* 1997 models: All models (except for the Honda Passport, Acura Integra Type-R, Acura SLX)

You receive:

1. At 50k-75k miles, you get a free emissions inspection and any repairs or replacements.
2. At 75k - 150k miles you get another free emissions inspection and you also get new spark plugs, ignition wires, distributor cap,
rotor and oil change for free.
3. Emissions warranty has been extended to 14 years
4. You can get reimbursed if you paid for any of the above emmissions parts or repairs.

It seems to be based on VIN so it doesn't matter if the car has changed a hands a bunch of time. Its apparently from a settlement
with EPA.
Here's a link to part of an official document at the epa.gov website on it:
http://www.epa.gov/OMS/url-fr/fr18jn98.pdf
If your vehicle has had this free service before (perhaps under a differant owner since this settlement actually dates back to
1998), than its not eligible. Call them up with VIN and they can find out.
The letter referred to in the SD post can be found over there, it was the letter sent to the guy posting. Its linked in such a way
that it seems to only work if you are viewing that thread over at SD. Apparently Honda is only just now sending out the letter to
vehicle owners registered with the applicible VIN numbers since the vehicles have entered the special extended emmissions warranty
and they figure that on average a 9 year old honda should have 75k-150k miles.
The 800 number in the letter for honda customer service is:
1-800-999-1009
Cliff notes: Call this number (or call your Honda dealer) with your VIN to see if you are eligible

Some more links that talk about this, apparently it was a $267 million settlement, at the time the largest ever with the EPA:
http://www.usdoj.gov/opa/pr/1998/June/264enr.htm.html
http://www.arb.ca.gov/newsrel/nr060898.htm
Looks like there was some flaw in the emissions computer that can make emissions worse, and honda didn't tell the goverment about
when it was getting certified.