Asa pointed to a bug for adding animated PNG support in Mozilla. A basic draft for animated PNGs is available for those interested. A sample image shows us that the format is backwards compatible. (I begin to appreciate backward compatible specifications more and more.) The advantage over using MNG seems to be that APNG is easier to implement and a lot smaller. Where you need 200KiB (or kB, not clear) for MNG, APNG is a 5KiB patch. If it is implemented Mozilla (and all products based on the core, like Firefox) all animated GIFs that are used for the product (internally) can be burned and eventually some sites will probably make use of it. Especially since it is backwards compatible.
The comments on Asa's entry are interesting, if also a little disappointing. Clearly there's a demand for this sort of thing in applications, and nothing would make me smile more than seeing GIF disappear from the Web entirely (well, not true; some other things would make me smile more, but that's not the point). And, since there's a demand for such a thing from more than one application vendor, it seems to me like discussion and eventual consensus are inevitable.
Anyway, vendor-specific muck is a non-issue in this case: the format is backwards-compatible. Sure, you don't get an animation, but I wouldn't get them anyway: I turn animation off.
Semi-transparent animations. Hmm. Sexyness.
I can't find anything about multi-frame capability of PNG on the official PNG sites though: only mentions of MNG. Please explain.
Gervase Markham sums it up real nicely.
All we want is an animated GIF replacement!
So MNG provides that. It can provide a lot more, too - but why is that a reason to actively reject it, if the size increase is so small?
What is the advantage of an Mozilla only specification (although open and backwards compatible) when the alternative only costs about 50kilobytes?
I assume MNG images are even more efficient than APNG images too, however I can't comment on that. (May be someone can compare throbber images compressed with APNG to a version compressed with MNG...)
The backwards compatibility is a huge advantage in my opinion. It is the reason we should use APNG I think and not MNG.