The Chip when run at 1ghz will consume 800 watts, yes you can get away running on a 1000w PSU however for continuous run time (24x7) you will be putting considerable amount of load on the 1000w PSU (80%) this will impact the efficiency, and will increment the risk of PSU burnout. When you compare it to running on a 1200 PSU you will be at 66.7% load which gives you efficiency and stable current . Which IMO if allows the unit to be run 24x7 with little risk.
A 1200W PSU seems excessive, but you might want one for overhead. I haven't pushed the Habanero up to 750GH/s yet, but the one test hashing right now at
http://eligius.st/~wizkid057/newstats/userstats.php/1CAqwaYmNn6SZbk4G3e9cXxsJ3tJVfGuzt is pulling
54.1A at 11.92V, or 645W. That last 10% might really push the power usage if you need lots of voltage, but I'd be surprised if you couldn't get away with a good 1000W supply.
Edit: I just went and measured it again as that seemed a little low, and the number is 59.1A. Sorry for the confusion.