Suzlon is using a hybrid tower for its 2.1 MW wind turbine. Its base is a lattice tower and above it sits the normal cylindrical (or conical) tower, for a total height of 120m. See http://www.suzlon.com/images/Enerasia.png. I have not found the exact dimensions of the lattice tower, but it looks to be about 40m in height.
At a high level, this is an inexpensive way to increase hub height and gain higher production. The claim I have seen here is that there is a 4 to 5% increase in wind speed, which amounts to 12 to 15% increase in annual energy production (AEP). Even if this claim is exaggerated, an increase of 10% AEP is a big deal with a modest amount of investment.
Update: Shooting for 170m hub height. For more details see "Wind Turbine Towers Poised To Reach ‘Mega’ Heights."