With the news spreading like wild fire on the internet, this rumor was quickly put to rest. It would be very difficult for this to happen since Spider-Man 4 should start production next year. The Hobbit will be 2 movies and will tie up the main actors for some time.
Dismissing the Spider-Bilbo nonsense, Tolkien fansite TheOneRing says: "Our sources indicate this rumour is NOT valid, again, this is NOT true according to our sources."
And, over at Movieline, Maguire's publicist screeched: ""This is false!"
I know some of you were getting excited that this might bring a new Spidey but it seems Tobey will still be web slinging.