Everyone in this thread is focusing on either the service/warranty aspect or acting like people are walking in for the sole purpose of flaunting iOS 7 when the OP asked about neither. I don't get it.
FYI, they cannot void your warranty for loading a beta software. There are consumer laws that protect us from crap like that. However,
they can refuse to service you until a currently supported version is loaded to prove the beta software is not the cause.
Ok, back on to the OP's post; I did walk in with mine while I was looking to buy some accessories. I was trying to use the Apple Store App to check out, but it kept failing. So I grabbed a rep and asked them to check me out since my Apple Store App was failing on iOS 7. The asked to see it and asked me how I liked it. That was about it. No big deal and I still wonder why people in this tread are making a big deal out of it.