The upgrade offer was available for Dell systems bought after June 26th, 2009, but the offer expired on Jan 31.     To get the upgrade, you had to first register your Best Buy system with Dell (which I assume you did), then sign up and register on the special Dell Win7  upgrade page.    I don't think Dell actively notified customers of the option, though it was relatively easy to find on the Dell site. A Dell liaison was offering help for those who missed the deadline, but it was only for a limited time, and I do not know if they still can.    See Dell-LisaG's post below - to send her a PM, just click her name, then "send a message". [URL]    
I was told by Best Buy that Dell would provide a free upgrade for my Dell system. In a chat room with a Dell representative, I was told that the offter for a free upgrade is no longer available. I purchased my system 7/25/2009. What is the real story??
Windows 7 upgrade