It is the simplest DIY project ever on TreeHugger, and its got pallets! Dave writes:
No-Cook Pallet Bike Rack
Instant bike rack made from two pallets -- no assembly needed. The pallets are not modified or secured, just leaned together. Seems stable enough. It's a little crowded for four bikes, but not unworkable.
The upper pallet (with top-side facing the bikes) creates the vertical slot that grabs the rear wheel and holds the bike up. The lower pallet (with top-side down) creates a 'pocket' for the wheel to sit in, so that the bike doesn't roll forward and wipe out. I figure there are other ways to keep the bike rolling if you only have one pallet, like a 2x4 on the ground, to work as a wheel block.
From these two pallets, the max width for tires appears to be 2", so wide knobbies would be a problem.
Locking securely might be a problem too. Thanks Dave, via Flickr.