Following other links from that site brought me to an interesting interview with Dave Brevik, who was one of the original creators of Diablo, and Diablo II.
The interview is actually about Hellgate: London, but the interviewer initially asks him questions about Diablo.
The most interesting point is where he's asked how he thinks the Diablo franchise is going, and he states "...I hope that they [Blizzard]
continue to work on it ..."
So it would seem that Diablo 3 was definitely started before Blizzard North was closed; but what they are currently doing with it is (obviously) a mystery.
Link to interview is here:
http://1up.com/do/feature?cId=3143557&did=1
and the referring page (with interesting editorialised comments):
http://www.diabloii.net/