[Galadriel]

That's a very interesting question. Personally, I feel Bilbo did NOT enter the 'wraith-world'. I think Tolkien would have mentioned it in one of his major works if the Ring had such a profound effect on Bilbo. Also, Sauron's power was still comparatively weak when Bilbo found the Ring, indeed I feel it was so till he left it with Frodo. And, as has been mentioned, Frodo was stabbed by a Morgul-blade before he put on the Ring, and this might have been an influenting factor.