And Andrew: The problem came up when trying to move my personal web site to a fresh install (it currently runs very old code from way before there was even the faintest notion of an upgrade path),. I tried several tricks to move data so the object_id's would stay the same, but failed. It seemed like the number of objects created during install was larger than it used to, so some ID numbers would already be taken. In any event, it's tough.
Don's idea of creating a new sequence per package ID should work, though it's not terribly pretty.
I guess the prettiest solution is Dave's suggestion of creating a unique short string from the summary to use in the URL, though that doesn't solve the "Bug #123" problem.