Here's a good guide to dealing with a nut-type bottom-bracket: http://www.utahmountainbiking.com/fix/crank2.htm

The BB spindle is actually threaded at the end:


And uses a 10x1.25mm serrated flanged nut:


These actually work better than the higher-end fixing-bolt and washer combination. Even when insufficient torque is applied, the serrations on the nut keeps it from working loose and falling out.