You're not alone santiagofrommiami and Viclr33, I too have the same problem with my iPhone 4 with iOS6 and my ID is on the Kuwaiti store since I'm from Kuwait. I guess it's an iOS6 bug that urgently needs to get fixed by apple!
My problem is that I can't download or update any app, when I try to, it says "Your account is not valid with the (Country) store....etc" Where country changes every now and then, sometimes US, UK or even Canadian!
I can't switch back to the Kuwaiti store whatsoever because I'm already on that store, and that happens with my brother too who owns a US id on an iPad3! Everything is working fine on my iOS5.1.1 iPad3 with the same id I use on my iPhone, and for that reason I still didn't update my iPad and wont until it gets fixed.
What I tried:
1- Backed up and restored my device.
2- Used another ID, with "none" as billing info, and then added that info, problem still exists!
3- Deleted every billing information and changed the country, changed it back to Kuwait and put down my billing info.
4- Used my friends ID's that are working fine on their iOS5 devices, but not on my iOS6 device.
5- Tried to change my info and write it back using iTunes, first time it worked for couple of hours, then stopped working and have been trying since then, can't get it to work.
Hope that what I've tried saves you time from going through any procedure you wanted to go through, it wont be fixed until apple fixes it, I guess!