One other possibility (I missed the earlier part of the thread) would be diatomaceous earth - it looks a little like sand, but is extremely sharp and happily carves slices out of fleas as they come into contact, so they dry up and die.
If the house has been vacant, the flea eggs may still be lying dormant, and will only hatch when they figure out there`s activity around them again.
If you are doing the house up and haven`t yet put carpeting down, it might be worth just sprinking some about to see if it helps.
Failing that, get a "sacrificial" cat and some decent flea killer (Frontline works great) and the problem should be resolved within a few weeks with any luck :-p