No way in h*ll I'd ever want to go that fast on a bicycle. Most of the reasons Dax had already explained in this thread. THe main reason is safety. I just have to think that other traffic - cars, trucks, buses, pedestrians, etc. - just to not expect a bicycle going that fast so they make their judgements based upon that assumption.
Another reason is that here in California it's illegal to ride a moped and especially a motorized, motor-driven cycle that fast.
Most of the cruisers, mountain bikes, and all of the cheapo wal-mart specials are clearly not designed for that speed. Full suspension may help but I still doubt that it can safely handle that kind of speed without a gas shock absorbing system of some kind.
WHen I was a kid I recall the trip over the handle bars when the front hub bearings failed because the axle nut came loose. I was doing about 10 mph. I don't even want to think about that happening doing 40+.
