I would say around 80% of the current Haswell certified PSUs are pre-Haswell. So, I guess most recent quality PSUs would work with Haswell.
However, there is of course no guarantee. But it should work fine.
I just saw a link:http://coolermaster-usa.com/press_release/haswell/haswell.html
Most power supplies don’t support Haswell – False
I need a Haswell certified power supply to build a Haswell system – False
Haswell only works with DC-DC Power Supplies – False
Haswell only works with 80+ Gold and better Power Supplies – False
Haswell requires a second 12V rail and doesn't work with single rail Power Supplies – False
On some older power supplies, Haswell consumes 5W more in idle mode - True
That's what they say...
Basically,certified,Haswell ready PSU means, that PSU sure will power up your Haswell system. But the others,there exist doubts...though it doesn't mean they won't...Just not completely sure...