What wallet were you using?
I'm using Bitcoin Core 64 bit. (now v0.9.2.1 - QT ver. 5.2.0)
Did you have it password protected?
If so was it a long, random password or a short common one?
Protected with passwd. 8 characters (each different, some lettes, some digits...etc.).. that's why I dont know how this could happened.
Relatively weak and easy to crack assuming someone got your encrypted wallet.dat file and could set it up in a dedicated machine to brute force it. Can you think of how anyone might have gotten your wallet.dat file?
(I use 32 character random passwords generated by LastPass).
Or it was just a keylogger. This is typically the cause of these kinds of thefts.