P.S. I love the idea of him being the narrator but its highly unlikely. For one, there isn't enough skeleton jokes for it to be him :P
After last weeks chapter chances are that Brook rather concentrates on improving his music than inventing more jokes, thus making him being narrator of one piece more likely =P
On Thriller bark brook got hurt fighting Ryuuma and was able to regenerate his bones (by drinking milk maybe).
http://www.mangareader.net/103-2566-15/one-piece/chapter-459.html
http://www.mangareader.net/103-2565-13/one-piece/chapter-458.html
Otherwise he wouldnt look that healthy today. (lol)
http://mangastream.com/read/one_piece/93580674/8
His skull somehow regenerated even if Brook stated that his body stopped growing which would also stop regenerating his bones.
Brook even got stabbed in the spine but survived, thus making the last vital organ unimportant.
We know four things:
1. Brook is just bones
2. Hurting his bones, even the spine, does not lead to death
3. Brook can regenerate his bones
4. Brook doesnt age anymore
If he doesnt age Brook can't get older and die.
If Brook doesnt have vital organs he cant die by physical damage.
The indications given in the manga lead to the conclusion that Brook is immortal.