well...following Apple's pattern with increasing cores...
1977 (June) - The Apple II used a single processor (1 x 1MHz).
1996 (Nov) - They started to introduce dual processors (2 x 200MHz).
2005 (Nov) - The PowerMac G5 had 2 x dual-core processors (4 x 2.5GHz).
2007 (Apr) - The Clovertown Mac Pro used 2 x quad-core (8 x 3.0GHz).
2009 (Mar) - The Nehalem Mac Pro now has 2 x quad-cores with hyper-threading (16 x 2.93GHz).
Following the pattern of the last 5 years...
2011 (February) - The x Mac Pro with 32 cores. Taking a quick look at the Intel roadmap, this will probably be the Sandy Bridge architecture. However that only goes up to 8 cores at the moment, meaning that Apple will need to introduce 4 processor chips to get up to 32 cores.
However this is all speculation and, as witnessed by this thread, will probably be wrong.
(Who knows, they could break through the 3GHz barrier to 4-5 GHz 😛)