I've found that 2 types of driftwood sink cordially when you put them in your tank: Malaysian and Swahala (African) driftwood. Tankgirl and I have the Swahala, and I and a lot of other members have the Malaysian, too.
However, there are few guarantees, as these old pieces of wood have often drifted around for quite a while at the water's surface: they are called "driftwood" for a reason, after all...
If the piece doesn't sink with time (I had to wait 3 months before one of my pieces finally gave in and sank), you can always attempt to anchor it with a piece of slate or with other sorts of weights. The slate can be siliconed (be careful to get aquarium-safe silicon) to the bottom of the driftwood, and then can be buried in the substrate so that the slate and silicon do not show. This is the most popular method, and you'll see many pieces on Ebay that have already been weighted with slate.