Also cant we some how automate the BC break testing for this (aka scan all functions that accept an array with the old API in 5.2, pass it an ArrayObject instance and see what happens and compare that to 5.3)?
Yes we can ;) That should be not hard to do by looking at ones having 'a' in parameters in 5.3 and comparing how they react to arrays in 5.2 and in 5.3. Some of them, btw, might accept objects even though it doesn't make much sense - as above.
This isn't the type of thing that I would naturally expect to find broken in a .x release, so if we can preserve the old behaviour with little to no penalty, I suggest we go for that.
S