I had the same issue.  It ended being something inside the transmission.  It may be the linkage like the others have suggested but I would get it checked out as soon as possible while you are under warranty. 

I hope you have better luck getting yours fixed than I did.  Mine first went in for servce on 3-15-2010.  It is now in the shop for the 4th time, at the third dealer.  The dealers cannot seem to fix it properly. 

The gear slipping issue has been fixed, but now they cant seem to keep the detents working properly.

Good Luck,

Another dealer horror story It is sad that the dealer can't fix such a simple transmission......