I was just talking about this with our friends this weekend. We range in age from late 20s to early 40s and were talking about how things are different now from when we were kids. Our little ones are toddlers (or one is due in August) so we were talking about what will things be like in 10 years from now. Most of us grew up in small towns. I know by 12 we were all out and about with our friends, riding our bikes all over town, some of us baby-sitting, some getting into trouble (mostly the boys being dare-devils) and only one of us could go to the mall (we didn't even have that option).
Personally, I don't think kids that young should be hanging out at the mall all alone. They don't need to focus all of their free time on consumerism, trying to show off their new clothes, and hanging out in cliques. Plus, there are so many other people there that it's hard to keep them safe.
However, I would encourage any outdoor activity, including a 10 minute walk to and from a location that you can easily find her. It sounds like you've laid the ground work for a responsible little girl. If you know the neighborhood is safe, she's allowed to be home alone, and you know (and agree with) the friends she hangs out with, then I don't see what the harm is. I would give her a time limit for the actual store, but would strongly encourage outdoor play. Not enough kids get outside and are losing out on crucial exercise. Be glad she's not glued to the TV playing video games or surfing online (which is a WAY more dangerous unsupervised activity than a 20 minute round-trip walk).
Also, I completely agree that you and your husband needs to be a united front. Do not let her know where you stand and dad doesn't. You guys need to decide what activities will be allowed and when a new activity comes up, you have to let her know that the both of you will discuss it and get back to her. That will discourage spur-of-the-moment outings that you don't know enough about and let you make an informed decision.
One last thing, she can get into trouble at home just as easily as when she's out and about. That is why you're doing a great job raising her to be trust-worthy and you're active in her life (i.e. you know her friends well). I'm sure this will be the first little bump in the teenage years. :-)