Except that in Ted the bear is actually the boy's life long toy that he's had since a kid, the toy is genuinely alive, and from what I can see the movie is is going to be based around Ted getting his own apartment and life.
Wilfred is about a dog that only 1 bloke actually can see, who essentially is just the personification of a dog in how he acts.
If we are going to say Ted is derivative of Wilfred, we might as well say that both are derivatives of You, Me and Dupree, Notting Hill and any number of movies that include a gross out room mate that interferes with the relationship of the main character.
As I said if anything is a rip off, Wilfred is a direct rip off of Brian from Family.