I was reading the 2010 NHRA rule book, looking for something else, and found on Pg 20.37 "general regulations", 10:5 "Driver restraint systems".
"in cases where the class does not require a SFI 16.1 driver restraint system, the two year recertification does not apply".
So most of us can run a "racing" belt, that is more then two years old. (basically, as long as we don't have to run a roll bar or cage)
Many part time racers do not carry a rule book with them, so I listed the reference page/section of the rule book, so if a tech inspector questions an "out dated" belt you can point it out in his book. I would suggest keeping this info in the car.