From what I understand, the reason why Ferrari hasn't used the DSG is due to copyright and patent infringements, along with VAG still being in talks about proprietary use of their transmission. The Bugatti Veyron has it, Porshes are soon to have it, Lamborghini is soon to have it, Ferrari is just hoping and developing their own solution (like most other car companies) yet not being like the original. Like Vista copying Leopard, close but not close enough.
DSG is not a VAG product though, it's BorgWarner's, VAG themselves have merely licensed it from them.
The system used by Ferrari had gradually evolved over the last 20 years, a system that was first designed for use in the Ferrari 639/640 (F189) in 1988/89 and first used on (any) production car with the F355 back in '98.
The shifting time of the system alone has almost halved since the introduction of the Enzo in 2002, from 150ms down to 80ms. I suspect that within the not too distant future, we'll see the F1 teams current seamless technology filter down into the FXX Evoluzione (or what ever model will eventually replace it), which will negate the argument between the two technologies, because as great as DSG is, a single clutch, seamless transmission will be greater, with the exponentially greater advantages of smaller physical dimensions, mechanical simplicity and significantly lower overall weight.