I am confused as to why you have rated this bug as "important." As you point out, there is no real way to make this determination in software without breaking the automated sid->etch migration process; the best that can be hoped for is an educated guess based on apt-policy, but even that could be wrong (for example, someone who prefers testing to unstable but has a crapload of unstable installed anyway).
It's something of a known deficiency, and I have no easy solution... nor do I think it's all that important to fix. YMMV, of course. Chris
