Coldest its been since owning the car this morning. About 10*F. Car started fine, immediately into cold idle and after a few seconds of running it backfired through the intake several times, or at least thats what it sounded like. Never touching the throttle. Ran fine after that and on the trip to work.

I've experienced this also. High load, low rpm pop. It can sometimes be replicated by coasting then stabbing the throttle when in 2nd or 3rd at 2,000rpm and below.

I've had the same problem happen to me only once at ~3,000 miles, during cold start. I'm now close to 5,000. Completely stock too.

I went to the dealership for an oil change and a routine inspection. While I was there they pointed out that I had this firecracker noise coming from my intake when the throttle was lifted off of idle. I had never heard this sound before but I had cleared a P0315 CEL a couple times.
To me it seemed that the CEL and the misfires were related so I upgraded all four coils and haven't had a problem since.

Only time any of my 20 odd cars backfired through the intake was caused by improper timing.... seems quite odd on a new car.

Injector malfunction? These Direct Injectors having seal issues?

I don't think it's a seal issue. Seems it's a no close issue. Probably not the direct injectors though, most likely (from experiance) it's the port injectors not fully closing.

yup, most of the intake backfires i have seen were a result of a leaking port injector. My M3 used to do it on warm starts.... if it sat turned off long enough the fuel system would loose pressure and the leak would stop and the gas that was sitting behind the closed valve would evaporate so it would only do it on warm starts.

Exhaust popping is due to over run fuel lighting off in the exhaust and is nothing to be concerned about. If you want to hear it more, get a high flow 2ndary cat and an axle back or catback exhaust.

The Teflon Seals on the Direct Injectors are leaking. Take it to the Dealership and have the tech call Technical Assistance / FTS. Toyota IS aware of the issue.

Which parts should I replace?

Take it to your dealer.

A friend's engine was recently inspected and the dealer found an injector o ring leaking fuel. He said heat could have caused it to melt or it may have been defective.

Mine also has the same issue. O-ring of 1st cylinder is completly gone. The picture is 1st and 3rd direct injectors. I just replaced all o-rings and insulators. I did not have chance to drive yet, but popping when cold start has gone.

Guys,
Are your popping sounds similar to mine?

[ame="http://www.youtube.com/watch?v=Ky_vDK4ePHI"]FR-S cold start - YouTube[/ame]