Same thing happened on my friend's Pista.
Only a month old and he was having the same problem...the LBS he bought it from probably didn't tighten it down hard enough or installed it incorrectly.
So, he took it back to them to have a look at it...they couldn't remedy the problem.
After a phone call to Bianchi, the LBS managed to have Bianchi send a new rear wheel since the bike was still under warranty.
Try to do the same...your bike should still be covered against these problems.