I made it into a tinyurl, if anyone wants to save space in their signature. I'm happy to keep the flag so long as I don't need the space for something else (like dragons). I don't suppose there's any chance of a teeny tiny Chicago flag, is there?
Here's the tinyurl code. You can copy and paste as is.
As for the thread subject - I think hiding the information in profiles is a bit too much of a hassle. I can understand the frustration of having to constantly say where you're from, but I think it's on the OP to somehow communicate that when relevant. I like the discussions of how etiquette works in different countries, but I think it's a fair assumption that a poster is North American, and likely from the US, on a forum based in the US with primarily US members - it's just math, you know? If I joined a forum based in the UK, with mostly members in the UK, I'd expect to have to specify that I was American when relevant.
I also agree that the US is big and varied enough that even here, there are huge differences from place to place. State flags might work if people actually recognized state flags, but as a Chicagoan, that still doesn't work for me. There's a HUGE cultural difference between Illinois and Chicago
I like the little flag idea, but I think the best approach is for OPs to try to be specific, and for responders to keep in mind that not everyone is going to have the same background they have.