This sounds exactly like the issue I'm having on my MB (see profile) - we're running different hardware, but I'm wondering if there might be an issue related to UEFI that is common between our situations.
In my case, I know that my Firewire PCI card is responsible (StarTech) - as soon as I remove the card, sleep/wake functions normally.
I'm running a dual boot system and the problem is identical in Windows, so I'm thinking it's a UEFI bug or a configuration issue.
Are you using any add-in cards aside from a dedicated graphics card?