In Vista/Win7, they mostly used glass background for parts of the window that didn't have any text on them. And for the few exceptions where that wasn't the case (like e.g. window titles) they added a kind of halo so that black text would always contrast with that regardless of what was below the window, or else just significantly darken the glass and use white text. All for obvious reasons.

Looking at the new iOS screenshots, I'm surprised that those reasons apparently aren't obvious to Apple designers.

Maybe I've dodged this by having a fully black background!

Hope they improve on some of these issues.

There's some instances of text illegibility that seems to be caused by buggy contrast detection and I think they'll probably fix that pretty easily since this is only the first beta. I think the readability concerns are really overblown.