An error of fact...
Hi Board
Triton's radius is NOT 1900 Km, but a mere 1350 Km...
http://www.seds.org/nineplanets/nineplanets/triton.html
As for a collisional origin, well maybe. Charon and Pluto are almost certainly the remnants of a fissioning process - Charon being the icy crust and Pluto being the core.
Guess we'll know more if the flyby probe ever dodges past Bush's slash and burn budgets.
Adam