I've researched maxillofacial matters for school, and have much experience in the subject.
What people are picking up on is the fact that Morrissey posess an underbite. This is due to excessive growth of the lower jaw; thus the reason for the massive chin, jutted lip, and lisp.
People who suffer from underbites, and this varies from extreme bite displacemnt to failry mild bite displacement, often suffer from a lisp, due to the teeth not hitting properly during speech.
This is quite apparrent in Morrissey's case. However, it's difficult to tell exactly how extreme his bite is, for I personally believe that he is somewhat self-conscious about it during appearances. I have seen pictures when his mouth is caught at rest, and it seems much more extreme at certain times than in others.
It is quite common for many patients to consciously try and keep their jaw set in a certain position when around others, in order to minimize the effect of the malalignment. Again, I feel this is readily apparrent by the way in which Morrissey seems to constantly place his hands in front of his chin during interviews, and frequently toys with his jaw; I.E., swirling his tongue along his cheek--possibly out of lack of confidence in allowing it to fully rest.
This may trivial to us, since many people see Morrissey as being very attractive, but to the patient it can be a very self-conscious matter.
If you look at Morrissey's profile, his features in general are what one could term, "Paleolithic." He has a very extreme skull structure, with a huge brow ridge, and a massive lower jaw. Interestingly enough, his upper jaw seems rather flat, causing his lower jaw protrusion to appear even more extreme.
Morrissey definitely has a lisp, which came across as being much more severe in his early interviews.
In concert he sometimes makes this extreme hissing sound when speaking, out of what I can only assume is self-mockery; or maybe him even trying to veil it through embellishment.
He has even joked in concert that he has trouble with S's