mba 13 2012 8 gig here w/ sleep issue

My 11 takes longer to wake from time to time, even when lid has only been shut a few minutes. I personally wouldn't worry. I think sometimes forums make everyone a little paranoid that something is wrong, when in fact all is well 🙂

I don't know if this is your issue, but I did read the other day that in Lion there have been some random black screen upon wake issues where the user logins did not show, just a black screen. In these cases if you moved the mouse cursor around it would reveal the login page under the black screen and then you could use it to sign in as normal.

It probably is sleep vs hibernate what you are experiencing
Everytime you close the lid and enter "sleep mode" the content of the RAM is copied to your HDD (hibernate)(suspend to disk) and the RAM remains in it's current state by still being supplied with power (sleep)(suspend to RAM).
So I would imagine Apple has a made it so that after 15 Minutes or so the RAM gets shut down as well ("User isn't coming back")
It's faster to resume from RAM (instantly) than to resume from HDD since in hibernate mode the RAM copy has to be written from HDD to RAM upon waking.