I have a New 09 G37 Sedan, about 2000 miles on it, and from 2nd to 3rd or 3rd to 4th it felt like the transmission may have slipped a bit a couple of times on the way to work. Going to test it out some more when I get off, normally I use premium unleaded, but I had someone fill up for me recently and they used regular.
Is bad gas a cause for a transmission to slip?

What do you mean by slipping? Is it coming out of gear? Is the clutch slipping? I've never heard of gas affecting the performance of the transmission. G/L.

Check the level of the transmission fluid right now. Not every new car is perfect, if you know what I mean. If the level is good, then try a nice 50% throttle acceleration run. It ought to snap through those gears pretty quick (I've never driven the 7 speed, but 50% throttle is exciting enough on the 5AT). If at 10% throttle you felt some hesitation, that's probably nothing. Maybe someone with the 7 speed has something intelligent to say...

You know I looked under the hood and I couldn't find a stick to check the transmission fluid, so I pulled out the ol manual...it suggests taking to dealership to check the fluid..
/sigh
What I meant by slipping was when it was switching gears seemed to spike in RPM a little more than normal.