This turned out to be an issue with the O2Micro SD/MMC drivers on the E6420/E6430 units that we had. After adding new drivers to our MDT server I started getting reports from users stating that their laptops were completely freezing up after waking from sleep - no response to keyboard, mouse, etc. No mini dumps were generated, powercfg - energy didn't show any major issues, and event viewer was useless. It was occuring both on Windows 7 and Windows 8.1 builds.
It wasn't until after I started disabling hardware components in Device Manager that I found a correlation between disabling the O2Micro SD/MMC controllers and it being able to wake from sleep. (I rebooted after each diagnostic test just to make sure all changes were in full effect)
Installing an older version of the driver and rebooting fixed the problem on all the laptops that were having the hang issue. Of course, just disabling the SD/MMC controllers is a fine fix too.