Hmm, I really think we should simply maintain BC 100% but using the new parameter parsing API and then really think this through for the next bigger release (presumably PHP 6.0) and come up with a set of interfaces for objects that allow them to more sensibly work with functions.
FWIW, I agree here. Yes, it's "wrong" now, but BC breaks like this in point releases upset a lot of people and discourage early adoption. I'd much prefer to see this in 6.0, especially if we can raise E_DEPRECATED in the meantime.
S