I have tried this using version 188.8.131.52 and get the field displaying: "--_-_-" with the input mask you mentioned. Only tested in Firefox 3.6.6. Probably fixed in the patch else it could be related to browser type/version and be a bug.
I have tested it on difference browsers and still cannot see the mask in the input element:
But I just found out that the client doesn't show the mask ONLY if we put the input element in the conditional formatting. This is than a bug.
So input mask works in a normal form and sorry I forgot to tell you that I use the conditional formatting in my form (did not expect that it will affect the mask behavior).