It happened to me today for the third time since I bought it. Not really and issue since it does not take any time waking up the machine, but it may affect the graphic card somehow.
I've reset the SMC and the PRAM, repaired permissions and check the disk for errors, just in case. I hope that after this it doesn't happen again, I'm going to call Apple tomorrow anyway and report back here.
I'm getting a little bit concern since there are other people experiencing the same. 😕
EDIT: After calling for support, they told me to repair permissions and do a Hardware Test. Nothing turned out wrong, I'm going to wait and keep using it, if it happens again then I will go for a check to the nearest authorized repair center. The main problem is that I can't find a way to repeat the behavior...I hope is not a hardware issue.