If you want packages from testing and unstable while running stable, I recommend to put APT pinning into effect so that one can do so in a controlled fashion - otherwise the system will invariably start taking packages from unstable as those are the latest versions and by default latest version gets highest priority. But with APT pinning one can still have the highest priority on stable yet still in many cases be able to install things from testing and unstable. Note though that sooner or later one will stumble upon things that cannot be installed unless a massive amount of packages will be upgraded to versions from testing or unstable. At that point one has to make a decision
A) Keep using the versions provided by stable
B) Effectively upgrade the system to testing or unstable
C) Attempt to find a backport of the package
Regarding APT pinning
And regarding the non-US one, that doesn't exist anymore since the release of 'sarge'.