There is no reason to use the distribution/branch name "stable" in any part of the apt configuration. Use the release codename e.g. "squeeze".
You should not be mixing the stable branch with the testing branch.
A mixed system: testing/unstable/experimental
- Use the branch names, e.g "testing", "unstable", "experimental" (you can use the codename "sid" in place of unstable because sid is always unstable, but use either one or the other throughout all apt configuration files).
- If you run testing, you can set it as the default release and install the odd package from unstable and experimental.
- If you run unstable, you can downgrade to, and pin/hold, packages from testing where necessary and install packages from experimental.
A stable system: stable (with the option of backports)
- Use the release codename e.g. "squeeze" - to avoid unplanned distribution upgrades...
- Apart from backports, do not mix other repos
If you're moving onto wheezy early, then treat it as a "stable system" - i.e. do not mix in unstable/experimental.