Yeah single rail. It isn't to say a multi-rail PSU can't be good but depending on the rail configuration there can be power left untapped.
I would consider this PSU to be pretty much unbeatable for making 3x5970 or 4x5970 rigs:http://www.newegg.com/Product/Product.aspx?Item=N82E16817151109
1250W is more than enough for 4x5970 or 3x5970 rig that you want to leave upgrade open. If you know you won't be going w/ 4x5970 1050W is equally good and saves some coins..
A staggering 104A on the 12V rail. Would like to point out that 1248W of the 1250W is available in 12V in one rail.
8 PCIe connectors so 4x5970 is possible without trying to fuck around w/ molex adapters.
A couple of minor points:
a) has oversized power switch which is nice (I set BIOS to turn on after AC loss and just use PSU switch to turn rigs on/off).
b) all cables are modular which reduces cable mess and makes swapping out PSU easy.
c) 5 year warranty and w/ SeaSonic you know who built it (SeaSonic) unlike other companies who just farm the work out.