As a followup to my previous post I've done some more experimenting with the mac mini.
I have 4 ram modules (2x2gb and 2x8gb) all four modules have been extensively tested (Memtest86+, Mac Server mem tester) for 30hours+ in two different machines.
Tested on a fresh install of OSX 10.8.3, no other software installed.
Results are:
2x2gb modules installed : no issues
1x8gb module : no issues (with either module)
1x2gb module + 1x8gb module (any of the modules, in any slot) : sleep / resume seems to work but eventually the display port monitor will flicker then go into sleep mode. Display port monitor recovers after power cycling the monitor, but will eventually flicker then go into sleep mode again.
2x8gb modules : Display port monitor blanks after system resume from sleep. Noticed on the fresh install (which was on an external firewire drive instead of the normal ssd) that the display port does wake for a second or so before going back to sleep. I wondered if maybe there was some disk activity briefly before it went back to sleep. Rebooting the machine is the only way to get the display port monitor working again in this case.
...
my conclusion is that this issue is occuring due to having > 8GB of ram. I'll probably buy 2x4GB modules at some stage to test it out.