I can wear just about anything under my jersey when road riding - tight camisole, shelf bras in tank tops, bralettes, etc. I prefer something that's decent if I unzip. I could probably wear a normal bra for my commute if I didn't have a brutal 1.5 miles of uphill that ended at my office (I have occasionally worn normal bras on the way home on that glorious, glorious downhill).

Even though I'm an A cup, I need a pretty hard core sports bra mountain biking. Most of the trails near here are covered in grapefruit sized rocks...kills my boobs and my bladder. (And have I gotten any better at picking lines? Not in the slightest).