This is kind of after the fact and all, but my wife's Macbook (second gen) developed a flicker that happened fairly infrequently, but definitely occurred. As with many things with her computers historically, she lived with it and didn't tell me about it so I didn't know about it until I observed it personally. It happened when her screen automatically dimmed when idle. I informed her that this was not normal and indicative that it might fail, and we should take it to the genius bar to be fixed before the warranty was up.
Naturally when actually at the genius bar I was unable to have it happen to demonstrate to the genius, but he informed me that they have tools that, if this was a bad inverter, they can use to invoke the behavior. They took it in, and three days later I picked it up with paperwork showing that they replaced the inverter.
So perhaps your genius bar isn't as well equipped as mine? Who knows. Hopefully your new Macbook will give you less flawed service, and whoever gets your old one will not suffer the flickery wrath of the LCD gods.