I would not "lock first posts by default" as Adywan and others have used the first post for general information, which is beyond helpful. However, "allow additional text, but original post text cannot be deleted" would be interesting, if you could work it out. That way we would be able to see what they first posted and what people are commenting about on that first page, but they would be able to have "9/19/2008 UPDATE" where they could add screenshots or links or whatever.

EDIT: This of course would not allow us to fix typos, rephrase ourselves unless we did something like this.

EDIT2: I should have put a slash between "typos" and "rephrase" or rephrased the whole thing. OR maybe I should think before I push "post". That might fix a lot of everyone's editing problems.