Keep an eye on the oil level, it very well could be burning oil.

 

Do you floor it often? It could just be soot buildup belching out, though that usually isn't white..

 

 

 

Keep an eye on fluid levels, that will be the best indicator that you are ingesting fluids.

Keep in mind this is spring time and you will get a substantial amount of condensation/evaporation coming out of the pipes until everything is up to temp. If you are still having this problem once the exhaust system is up to temperature then you are probably consuming something, it's really hard to tell from the driver seat exactly what it is but if you can get someone else to drive your vehicle and reproduce the issue while you follow it's very easy to tell if you are burning oil because of the smell.

 

When you did all that coolant system work, are you sure the coolant was properly bled? If there is any air bubble in the system, you might notice temperature instability or overheating, or loss of heat from the heater. The G35 coolant is highly sensitive to any air in the system and will behave weird if not bled all the way.

When I replaced my radiator and refilled with coolant, I still had an air pocket and I was making some white smoke with throttle. I was very afraid it was a head gasket since I had bought the car with a cracked radiator. I bought one of those radiator filler funnels that allow you to put a few inches of coolant above the rad cap for proper bleeding. After a few thermostat cycles and hose burping with the bleeding funnel, the temperature stabilized, my heat came back, and the white smoke went away.

 

White smoke normally indicates a blown head gasket, keep a very close eye on both oil and radiator fluids! And follow what Maschinenbau suggests too, making sure there's no air in your coolant system!
Gary