Well, just look at the Istambul specs, it uses an only DDR2 IMC, due to backward compatibility... AMD claims to have full DDR3 support for Opteron systems with his new platform (new C32/G34 sockets on 2010, the Fiorano chipsets SR5890 + SP5100 still on DDR2 memory) and Lisbon processor is the six-core update for this new platform.

Now we know Lisbon situation, we can read this:

If Thuban exists, it could be a derivative of not Istanbul, but Lisbon, AMD's next-gen six-core offering.
Link.

So my conclusion is, Thuban's IMC -> DDR3.