Yeah, in one interview someone asked inwhich animagus Harry will transfrom; and she answered that Harry wouldn't become ananimagus like his father. (I'll have to find the quote)
No, he isn't a dead omen, that was just Harry's over imagination after seeing the one in the book; I think that Sirius is a dog because dogs are loyal, strong, intelligent, and can be very agressive, but also very careful and good behave.
I mean, I know that Sirius isnt' a death omen, but strange taht his animagus form is so similar to one... *and Harry said that he looked just like the grim... *
I dont' know... it just seems to be a kind of strange thing to keep repeating... JK calls him a bear sized dog lots of times...
kinda like she's trying to hint at something...