Hm, I said I'd dig for info on whether Oda said One Piece was tangible or not, but I'm simply too tired right now and need to sleep (been up 25 hours writing and submitting my 15 page final / research paper -_-

In any case, this is what I've found so far:

In an interview, Oda is asked:
Q.37 is there a secret which no one around knows?
to which Oda answers:
A The secret of the naming of the title One Piece.

This leads me to believe that he has not dropped any direct-answer type clues about the nature of the pirate treasure One Piece.

But in any case, sabret00the of AP forums claims that "Oda has specifically stated that One Piece exists and is tangible." in this thread (post #11) and doesn't point to exactly where except to mention that it is in an SBS. (post #30).

I've run through the SBS volumes (4-43) briefly once but didn't find anything, so I was going to go through them again and read each question carefully. I'm up to volume 11 now, and I'm going to sleep =P