A few points of evidence aim towards Diablo 3.
Blizzard never do teaser announcements or build ups to any of their expansions.
Blizzard very recently took over the domain name of Diablo3.com
There are 6 days remaining until June 29th: The last day of the invitational, the date both Diablo 2 and Lord of Destruction was released in 2000 and 2001.
There is no logical reason to tease something they have already announced (they have never done so before)
Hell hath frozen over...
Diablo 2 also had lots of runes, not just Arthas' sword
But who knows except Blizzard? We shall all see in 6 days eh? ;-)