Enroute Controller described a loss of separation event when traffic was issued holding instructions awaiting a 'landing assured report'; experienced a fuel issue; and descended below assigned altitude.

Narrative

Aircraft Y was observed below 3;000 FT on ILS approach into the airport. Aircraft X was also inbound to the airport and advised he was #2 behind aircraft already on approach and to expect approach clearance upon receipt of the first aircraft cancelling IFR. I issued a descent to 4;000 FT and holding instructions to Aircraft X at the intersection to hold southwest on the localizer course. Aircraft X advised 'minimum fuel.' He then advised that the aircraft ahead of him was on the ground; trying to cancel on the UNICOM frequency. I asked Aircraft X to tell Aircraft Y to come to my frequency and cancel and Aircraft X said; 'I'm too busy.' That's when I noticed he was descending through 3;400 FT and had not entered holding. Aircraft Y cancelled IFR on my frequency and I then tried to clear Aircraft X for approach; but he did not respond and was level at 3;000 FT flying towards the airport. He then requested vectors to the localizer and started climbing on his own back to 4;000 FT. I issued vectors and he joined the localizer; flew the approach and landed.