I had the same problem about 6 months ago with my older 15" Macbook Pro (the aluminum one without the black keys, essentially the model before yours). I have had the same issue during that time and after a few months my screen went pixelated and I couldn't access anything. I took it to my local Apple certified retailer and they had told me it's a common problem with the older macbook pro versions. Essentailly the graphics cards crapped out on mine. Luckily there was a common issue and at that time Apple covered that under an extended warranty ( you have to remember I have had this macbook for almost 4 years now and I had never purchased Apple Care). So basically the retailer (Small Dog Electronics) sent the macbook out and had Apple replace my graphics card for free. It was a $400 expense that was not charged to me.
However, now the same problem is occurring again. I have these evenly spaced "blotches" on my screen. This occurred as I took it out of my backpack from class and had restarted my mac just to refresh it from such prolonged use over the course of my exam studies. I'm not sure what's going on again, but I'll have to go to Small Dog Electronics again to see what's going on. I'm hoping that the graphics card does not burn out again or else I'd have to pay. It would be lovely if Apple covered it again at no cost, considering they replaced it already and it might fry in a few months already. I'll keep ya posted and you let me know the same.