Stores vary wildly. My local shop took the slip from me but had no idea what to do with it and probably just tossed it. The next place I went actually told me to hang onto it and try it at another store. The third was a HobbyTown, and the beardy dude they had working the gaming section actually took my driver's license and recorded it along with the code on a log sheet. He seemed to take it super seriously. I stopped at a fourth place on the way home yesterday, and he took the printout but nothing else.
I think HobbyTown must have some weird policies, or something. They *clearly* don't sell a lot of MTG. Hell, there were unopened packs of Kamigawa, Scourge and Shards of Alara just sitting in the case next to the register.
And yes, Modern is super fun. I love Standard, but they play out a lot differently. I'm constantly having to switch decks in Standard to keep current, whereas you can really focus on a specific style in Modern and learn every single trick.