Layers work for me. On the 2006 Spring Solvang Double I wore:

shorts
short sleeved jersey
wool arm and leg warmers
gortex waterproof jacket with hood (love the hood)
performance illuminite jacket (probably useless)
windbreaker vest
buff headband
full booties (because I hate soggy feet)

I was only cold when I wasn't moving. That is the key. I was the coldest at the mile 80 stop, but I knew the fix was to get moving again. By the time I made it to Morrow Bay the weather was starting to clear.

Btw, fixing flats in the rain sucks.

The best equipped people were running fenders. Those are the people you want to draft.