In order to remove the last of the veto callbacks (the only one used, in only one place), I went ahead and rewrote SPDocument::getObjectByRepr() so it didn't rely on ID lookups, then removed the change attribute veto callback.
Two side-effects of this:
1. it is now possible to remove the id attribute from an element
This shouldn't cause any problems, as I laid the groundwork for id-less operation a long time ago (this has been planned for a while). Note that objects still get ids by default.
2. manually setting a conflicting id now causes the original object to get a new id, rather than disallowing the change
This should always have been at least possible through the XML editor. I'm not as sure about the properties dialog; if we decide it's best to disallow setting a conflicting ID again we can implement a check in the dialog.
I think both changes are more in line with user expectations anyway.
Test and let me know if I missed any breakage. I tried to be conservative about the change.
-mental