AT&T locked Streak - who to call for unlock code?

​I got a Streak last November from BestBuy, and I suppose it's AT&T locked (never tried another card, though). Now, I am about to travel to Europe, and would like to pop in local SIM card.​

​Who I am supposed to call for unlock code? Dell? AT&T? Best Buy?​

​On my previous encounter, AT&T were happy to provide an unlock for AT&T Tilt (HTC TyTN II) which worked quite well in Germany.​

​Thank you for everyone for details on that (I hope I am not the first one to ask this question).​