The SLI-certified/ready and Crossfire-certified/ready are equally meaningless. A vendor can send in their PSU to either AMD or nVidia who look at it, call it pretty, slap a sticker on it and send it back. Look through the list of SLI-certified and crossfire-certified PSUs, most of them should never run more than a pair of 6450's in crossfire.
Take this unit for examplehttp://www.newegg.com/Product/Product.aspx?Item=N82E16817338008
SLI ready and crossfire ready, probably wouldn't survive it's first experience with either. With 20A on it's 12 V rail, my 955 and 4850 would push it over it's limit, let alone crossfire or sli.