Mine started flickering after 11 months from new. As soon as the gray apple logo screen came up, it would flicker really badly. At first it was barely noticeable, but take it from me, this problem gets worse with time.
Luckily I had a month left on my warranty so I logged a call, booked a genius and took my FlickerBook in to my local Apple Store. While I was there I pointed out the other, minor, niggly things that were wrong with it. They had it for 2 days before giving me a call to pick it up.
Turns out it was a power inverter issue (or something like that) which they replaced. They also replaced my screen surround (it had never fit snuggly with the lid), replaced my entire keyboard/trackpad/top piece (it was dented when I first took delivery of the MacBook but I never bothered taking it back) and did something to quieten my fan.
The upshot is I got a nice new clean keyboard with no staining on the mouse pad, a mouse click button that actually worked, quieter fan and a fixed screen. Happy flippin' days!
So glad the flickering started within the warranty period!