Personally I don't think Dumbledore will be coming back. It would be a bit of a poor ending if he somehow came back to life. Might as well just bring everyone back to life, like Harry's parents. Bit of a cop out really
I'm not so convinced that Sirius Black is dead to be honest. His "death" was a bit vague I think - I expect him to make an appearance in book 7.
Finally I am convinced that Snape isn't a bad guy. It would make good book writing for JK Rowling to try to make everyone believe that Snape was a bad guy all along, but in actual fact it turns out that he is actually good. Dumbledore never stated why he trusted Snape so absolutely, but I think he was right. Dumbledore was one of the wisest and greatest wizards ever - he wouldn't make a mistake as big as that
