OMG, I had the same thing on my core duo MacBook when I first took it out of the box. I seriously thought the screen was messed up or something.
Here's my theory, I think it's from the protector that goes between the screen and keyboard. It like wears a film off onto the screen. When the screen's off, or really dark, from certain angles you can see the streaks/lines.
Then I made the mistake of taking a microfiber cloth and trying to wipe them off. This only made it look worse, IMO.
I'm glad to say though, that I now have no such marks on my LCD. They wore off in the first 2 weeks, probably from the heat/use of the LCD.
Enjoy your MacBook!
EDIT: You're right, they did appear to be "inside" the screen, but either they weren't, or they're just how the LCD comes and using it breaks it in any goes away. I don't really know, all I know is that they definitely went away. Little things bother me, and they were driving me crazy. I was so glad when they disappeared.