It is clocked lower due to heat concerns, there is no ATX spec limit on the amount of power a single gpu can draw(hence the reason for pcie connectors, such as the GTX285 classified with three pcie power connectors on it).
Anand, among others, reference the card near hitting the 300W atx spec limit in its retail underclocked state, and selling fully clocked would be non-compliant. Of course, in time, that value can go up in the advent of new pci-e connectors specced for higher wattage, as it already has a couple times.
In order to overclock the 5890 to its "default state" (
5870x2) the power supply has to run the lines out of spec, as 75w from the pci-e slot + 75W from the 6-Pin, and 150W from the 8-pin = only 300W. Not all power supplies can handle being run "out of spec". It doesn't really matter anyways, as the VRM's seem to be the limiting factor, not heat or power supply capability, as the heatsink is supposed to be quite capable.
I haven't done a whole lot of research on the 285 classified, but it only has 3 6-pin connectors, which provide the same wattage as 1 6-pin and 1 8-pin (300W). Why they went with 3 connectors for the same wattage, I have no clue. I can only think they were thinking the connectors would be on separate rails so it would be a bit more stable, but most power supplies (decent) run one big rail now anyway.
Anyway, I'd like one of these, lol, as I'm sure we all would. But I'd like some eyefinity action personally.