@David: You could probably get through the Darien Gap without *too* much trouble on a mountain bike, but considering the terrain and dangers involved, I sure as heck would hire a ferry to boat me around. I'm sure that's what they're doing.
If they attempt it, this project is suddenly a *lot* more impressive. Crossing the Darien Gap safely overland would be, IMO, more impressive than the rest of the bike trip combined.