I rode this morning with temps about 38 or so:
lightweight Patagonia capilene short~sleeve T
long sleeve silkweight capilene shirt
windproof Marmot top
padded shorts
long, windproof pants
socks
shoes
fingerless gloves
helmet

My ears and fingers were a bit cold, but otherwise I was comfy (and I tend to get very cold very easily).
I think the trick is to layer and wear some good wicking clothing so your sweat doesn't get trapped and make you cold. The capilene stuff is great for keeping the sweat off. Also, it's amazing what windproof clothing can do to keep you warm.
Hope this helps~ Cheers!
Teresa