NVIDIA now certifies SLI parts and systems with its new logo programme. Actually there are two programmes, but they do roughly the same thing from different angles. The first is the SLI Ready program, and it is for parts. It covers graphics cards, motherboards and power supplies. The other one is dubbed "SLI Certified", and it is for complete systems.
The SLI Ready logo means the component in question was sent to Nvidia, and it got some poor intern who annoyed the wrong person to test them all together. Lots of cross testing, but in the end, if you go out and buy the logo'd pieces, they should play well together. That is a good thing.