I've had the same problem twice - once about a year ago, then again this past Tuesday morning. The first time I wasn't able to do anything about it, but this time I had it towed to the dealer (who has successfully failed at almost everything they've attempted before). The semi-short version is that they said the air filter was dirty (I went and looked at the filter - it was surprisingly dirty, but nothing extreme), causing low air flow. The tech said that this caused the engine to suck oil from the crankcase into the combustion chamber, causing the smoke and noise.
I'll keep it short and just say I'm not too impressed with their answer for a lot of different reasons. It occurs to me that they may be trying to establish fault for any future engine failure, saying that the damage was caused by my having a dirty air filter. Not sure if they have anything to gain by doing this, as I assume they would get paid well by Infiniti if they ever had to do warranty work on the valvetrain.
Some research on other forums shows that this is a fairly common occurence with FX45s. Haven't found anyone who had the problem identified and resolved by the dealer. My assumption is that it is a sticking valve or lifter. Regardless, it is definitely metal-on-metal, and I have no doubt it has added the equivalent of tens of thousands of miles to the engine wear each time it happens.
cpp