I had the _exact_ same problem a few times during the nasty cold snap in MN recently. The first couple of times, it just warmed up and was fine. The last time, it continued to not rev past 2K in -25F weather, even once it was warm. I discovered that by revving the engine (once warm), then dropping it back to idle repeatedly, I was able to up the RPMs by 100-200 each time. I did this until it looked like it was responding well enough and took off.

I drove through city streets at about 30-35mph without any problem, but when I hit the freeway after another 10 minutes, I found another limitation at 4000 RPM. I pulled off and repeated the revving until it cleared up and was able to jump to 6K without a problem. The weather has since warmed up in Minnesota and it hasn't reoccurred.

I have heard suggestions that the cold might be messing with a sensor which would be causing the computer to set a rev limit. No idea how valid that is. Any ideas?