Toyota made the decision to offer an extended warranty after seeing what it calls a "small number" of Tacoma trucks with excessive frame corrosion. Some news reports hint that a large number of trucks are affected, and that Toyota didn't provide nearly enough rust protection when those trucks were built. Most of the affected trucks are from states that use salt to de-ice roads in the winter.
If your truck is affected, take it to a Toyota dealer for inspection. If rust perforation is confirmed, Toyota will repair the truck or buy it from you. If no rust perforation is found, the truck will still be covered under the extended warranty for 15 years from its original sale.
It isn't unusual to see surface rust on a frame, but take the truck to a dealer to find out how extensive it is. If you aren't satisfied with Toyota's inspection, get another opinion.
Notification letters have been sent to owners. Toyota emphasizes that the warranty extension is a goodwill gesture from Customer Service, not a recall or special service campaign. If you have questions, call the Toyota Customer Experience Center at 1-800-331-4331.