Dear Subaru Owner:
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> We would like to thank you for your selection of a Subaru vehicle. At Subaru, we take pride in our products and are committed to your continued satisfaction.
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> Subaru has developed a modification to the Engine Control Module software logic to improve the monitoring parameters within the On-Board Diagnostic (OBDll) system for the engine coolant thermostat. This correction ensures that should there be an abnormal condition (i.e., the engine coolant does not reach or maintain the proper operating temperature), the “check engine light” will illuminate. The proper illumination of the “check engine light” is needed to alert you that corrective action may be required.
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> This software change has no effect on fuel economy or vehicle performance.
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> WHAT WE ARE ASKING YOU TO DO PROMPTLY
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> We ask that you schedule an appointment with your Subaru Dealer as soon as possible to have your vehicle’s Engine Control Module software reprogrammed. This will take approximately 30 minutes to complete. However, it may be necessary for you to leave your car longer on the day of your appointment to allow your dealer flexibility in scheduling. Your Subaru dealer will perform this modification at no cost to you.
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> CALIFORNIA REGISTERED OWNERS
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> The California Air Resources Board requires that emission related campaigns be completed prior to California’s vehicle registration renewal process; without this repair, you will not be able to register your vehicle during your next annual registration. Upon completion of this campaign, your California dealer will complete and provide you a “Proof of Correction Certificate.” If required, present the certificate to the California Department of Motor Vehicles (the “DMV&rdquo when renewing your California registration as proof of campaign completion. If the DMV does not request the certificate, we recommend that you keep it for your records.
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> In addition, the State of California requires that every vehicle must pass an emission test (SMOG Check) every two years and before it is sold. Without the service we are providing at no charge, your vehicle may not pass this test.
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> IF YOU NEED FURTHER ASSISTANCE
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> To locate the nearest Subaru Dealer you can access our website at www.subaru.com and select “Find a Dealer”.
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> For additional information and the most Frequently Asked Questions, please go to: http://www.wqc39.service-campaign.com
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> If you need additional assistance, please contact us directly:
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> Telephone: 1-800-SUBARU3 (1-800-782-2783)
> Monday through Thursday between 7:30 a.m. and 8:00 p.m. ET
> Friday between 10:30 a.m. and 5:00 p.m. ET
> Saturday between 9:00 a.m. and 3:30 p.m. ET
> E-mail: Go to www.subaru.com and select “Contact Us”.
> U.S. Postal mail: Write us at Subaru of America, Inc., Attn: CDS Department, P.O. Box 6000, Cherry Hill, NJ 08034-6000.
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> Please contact us immediately if the dealer fails or is unable to make the necessary repairs free of charge.
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> Your continued satisfaction with Subaru is important to us. This update is intended to ensure your vehicle’s optimal performance. We appreciate your understanding in this matter and regret any inconvenience this may cause you.
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> Sincerely,
> Subaru of America, Inc.

As apetron states, the dealer does hand out a slip to prove you completed the reflash. If your car is modified, DO NOT get the reflash, ABSOLUTELY DO NOT. I did a reflash once and the car ended up not being able to idle properly and stalled constantly. I popped open the hood to find an aftermarket cold air intake and with the MAF sensor installed upside down (MAf sensor at bottom of pipe instead of top, I assume it's the wrong intake for his car). The car came in running perfectly fine, but after the reflash it just crapped out, so I know it's because of the intake. Subaru programmed this reflash assuming your vehicle is stock, so if you're modified, consider yourself out of luck for the dealer reflashing your car. Maybe you can get your car tuned after the reflash?