I have also the same issue. I have updated to BIOS 1503 (Asus P8P67 Deluxe Rev. B3). I am planning to re-install OS (Windows 7 SP1 64-bit) to see if the issue exists also with a "clean" OS install without any Asus drivers. Unfortunately, I don't have time to do re-install now.
I made a little bit research, and it turns out that Windows 7 Service Pack 1 contains a bug related to sleep functionality when SCSIport minidriver is used. Apparently Marvell 91xx SATA 6G Controller uses this driver. I switched my hard disk from Marvell Controller port to Intel SATA 6G Controller port, and now I am able to put my computer into sleep without problems!