I can't see Snape being totally evil though....I also believe that Snape killed Dumbledore to have the "Unbreakable Vow" come to pass (ie. its no longer in effect?). I think Snape and Dumbledore do have something going on in the background (along with everyone else it seems
I think that maybe Snape's heart is in the right place (on the good side) however his methods are much more abrasive because of the life he has had. He was constantly teased as a kid at Hogwarts by James Potter and from the sounds of it since James was popular chances are he wasn't well liked with the rest of the school. I think he does what he has to to survive, and in the case of Dumbledore's death it would be used as leverage to be more trusted within the Death Eaters (especially since he's been "in Dumbledore's pocket" for 15 years).
I think Dumbledore is dead. The freezing charm put on Harry would only be released by Dumbledore saying as much or his death. Dumbledore would NOT have invested in Horcruxes and unless there is another way to keep yourself alive without splitting your soul....what I WOULD like to know is what happens to Fawkes, Dumbledore's Phoenix. He DID come to Harry's aid in the second book so will he make yet another appearance now that Dumbledore's gone?
mugglenet.com has a list of things Rowling should be revealing in the last book and some bits and pieces that Rowling's mentioned will happen. Has a bunch of other things on there too.