Well there is the description in his bio and everything, but I took a close note about Buck back from ODST.
* Buck couldn't push a brute chief off of him, unless he is playing weak.
My best guess,
Like Johnson, I think Buck may have been one of the candidates for the Spartan I project. Where we see Johnson and know something has to be more of a combative edge to the man, but Johnson doesn't portray any huge feats of strength like Master Chief does. Also Buck is 6'1" and Spartan II are much taller. I do have a lot of reason to believe that Buck could be one of the Spartan I test subjects.
Additionally, like many others, we all know Buck has to have some role in Reach. There have also been a lot of speculation about the comparison of Halo: Reach's Noble 6 to The 7 Samurai. The 7th Samurai was not an actual Samurai, but wanted to be one. I love the direction that the writers of bungie have taken to making allegories like Halo 3: ODST was (In my opinion) a very well written reference to Dante's Inferno (especially with the nickname to ODST are Helljumpers, such a great idea).
Who knows? The Noble 6 could be an allegory to the 7 Samurai. Buck could be one of the Spartan I test subjects. Like the 7th Samurai who wished to be one, Buck could possibly be that one Spartan, since the Spartan I were technically a failed project.
Or of course, Buck could just be a cameo appearance in Reach so we know what he did and how he survived Reach. I think you have to be a bit skeptical with the easter eggs and teasers Bungie puts everywhere though.