In reply to The Lemming:
When i was learning, my instructor told me about the last time he went out on his bike in a tshirt and jeans...
A bee flew up the arm of his tshirt - since then hes always worn his jacket.
So even if you dont come off, there are dangers :oP (not as bad, but still)
I always wear my jacket.
To get to college I usually just wear jeans (no where to change at school, plus its about 5 mins down slow roads).
If I'm going anywhere else I wear my proper trousers.
At the local bike shop you can get jeans with kevlar in them or something.. I would get a pair, but they are all mens style and dont fit!!