I forgot to mention that SickBikeParts has 132mm and 153mm cartridge BBs at reasonable prices. They're nicely made units, too. The crank you posted a link to does not look like it has too much flare in the arms, so you might be able to use such a long BB and not even have to pedal like a duck.
It make sense that if you're reserving your pedal efforts for high speeds, you'll need very high gearing (especially with a 20" wheel). There's an unacknowledged benefit to huge rings, too-- they are much easier on your chain, so the chain and sprockets can last a long time. As long as you understand that short crank plus big ring will limit the amount of power you can contribute at the pedals in most situations, and that the cranks may have to be placed on a longer BB spindle than would otherwise be necessary, there should be no other normal operating drawbacks I can think of.
The clearance between chainring and ground will be much diminished due to the larger ring and lower BB height. A 60t ring has a radius of almost 5 inches, and your bike rolling on 20" wheels could have a BB height of only 8 inches. So you'll need to take special care rolling it up and down steps and curbs, lest you smash the ring into a hard surface and damage it.
If you want to use a 48-60 double combination (a pretty good idea in my opinion), a "road triple" front derailleur would be a good choice. Road triples are usually configured with 30-42-52 rings, so the matching front derailleurs are best matched to the curvature of a 52 to 54t ring but with a 22t difference. That means you would have to run the derailleur up a little higher off the teeth of a 60T ring to keep from fouling the tooth tips, but 12t to 16t difference should not be a problem at all.
Here's one example that should work well and not cost too much:
http://www.amazon.com/Shimano-8-speed-Triple-Bicycle-Derailleur/dp/B003NV9AI6
Chalo
