There will probably be item sets in Diablo III, but what form they will take is still under debate by the D3 Team. The only official information we've yet seen about them came from Jay Wilson, in a December 2008 interview with 1up.com.
- Jay Wilson: There're still a few things that we haven't made decisions on yet -- set items, for one. I didn't like the way they worked in Diablo 2, as by the time you finally got a set together, you generally leveled beyond the use for it. So you might save them for alts, which is OK, but I'd rather that they be useful for you to begin with. We haven't really decided how we're going to fix that.
More information will be added when it's available.
There was a green-colored items seen in the BlizzCon demo in October 2008, but it was a quest item; King Leoric's Crown, which was used to awake the Skeleton King before the final battle of the BlizzCon demo build. The crown could not be equipped, just carried and then used to call the King's spirit back into his bones.