I've never been able to resume from sleep when using my 'Overclock' profile....my system would go into sleep ok but when trying to resume, everything would fire up and all the keyboard and mouse lights would come on but no matter what I did I couldn't wake the monitor up. This only happened in the 'Overclock' profile...using the standard profile sleep worked fine, then I saw this on the Asus support site.


Solution
Please make sure "Internal PLL Overvoltage" item in the BIOS setup is disabled. Enabling this option allows better Turbo Ratio overclocking capability. However, the side effect is not being able to resume from S3. Therefore, it is recommended to disable this option if you need to use S3 function, or if you are not going to overclock any higher than 47x in Turbo Ratio.

It was under 'support' for P8z68 motherboards (mine is...and my overclock is no more than 4.7Ghz) so I tried disabling 'Internal PLL Overvoltage' as Asus suggests and bingo it resumes from sleep all the time
I just thought I'd post this here in case anyone else is having the same trouble or in case someone may know a reason why I shouldn't disable this setting.