Men’s Journal shows off the Delta 7 Sports’ Arantix mountain bike in its Sept. 2008 issue.  The frame uses an open lattice, carbon-kevlar “Iso-Truss” design, as in trusses made in the shapes of isosceles triangles.  Frame weight is a very light 2.75lbs.  While just featured in MJ, this bike was actually shown at Interbike back in 2007.  There are probably two reasons you’ve never heard of it between now and then, take your pick:
1. There are only about 200 frames made per year, so it’s unlikely one will show up at the trailhead.
2. It’s a $7,000 exercise in engineering that’s impractical on trails with dirt, dust, mud or water (ie. Mountain Bike Trails).
Actually, on their website, they show the downtube with a condom-like wrap around it.  The technology is really cool, the frame is supposedly indestructible, and it’s sick light.  If you’re interested, it’s available with either a SRAM or Shimano build kit.  Each frame uses over 1,678 feet of carbon fiber and takes about 300 hours to build (hence the limited build quantity.
Gizmodo has some video of it here.
