The "We'll be back" rule is on page 18 of Chapter Approved. I have no idea if it will be altered when the codex comes out in a couple of months, but I suspect the only change will be to include new units in the rule.
Basicly Instant Death (as Unforgotten_Caste stated) and power weapons in close combat.
Also remember that there must be a functioning necron model of the same type with in 6" of the downed model for the the player to attempt the auto-repair. So if you can manage to wipe an entire unit of warriors, and the next closest unit of warriors is more than 6" away, then there is no possibility for self repair. The Necron Lord is the exception, it always get to try reguardless of how far away other units are.