Underwear defeats the purpose of the chamois in the shorts and the wicking ability of the fabric.
If people want to wear underwear when they ride, let 'em. But I'll never understand why they'd wear it under lycra cycling shorts. If you're going to wear underwear, the cycling shorts do you no good.
Training: 2002 Fuji Roubaix Pro (105 triple)
Commuting/Daytripping: 2001 Airborne Carpe Diem (Ultegra/XTR, touring wheels)
Commuting/Touring: 2000 Novara Randonee (Sora/Tiagra/LX, fenders, lights)