Nice TT time pedex!
I usually wear a helmet on the mtb. Oddly, I'll wear it on the road if I'm riding near the middle of the day and it's a hot muggy day. The helmet gives some shade (low hair count) while still allowing good air flow. If it's not baking hot, the sun is low, or I'm riding where there are a-lot of insects flying about (common around here), then I'm more apt to wear a cycling cap. I really like cycling caps - one of the simplest and most functional pieces of clothing ever developed.
I'll throw in my two cents on the Lycra/chamois thing. I think they are a little more comfortable than some other shorts though it is certainly possible to ride comfortably even in blue jeans. I find cycling gear decidedly uncomfortable when off the bike.