I've been layering multiple thin or medium base layers. I get synthetic ones from REI. I'll often have two base layers with a short sleeve jersey on top.

My PI convertible jacket, with zip off sleeves, is very breathable and has a huge vent across the back -- so if I unzip part way, I get lots of ventilation on climbs. This is workable down to almost 30F, but I don't like riding when it's that cold. It's perfect for mid to upper 40s.

The short sleeve jersey and two base layers without the jacket is great for warmer days in the 50s. Each base doesn't block wind, but the layering makes the set work quite well. This is warm and very breathable, and I like the flexibility of multiple layers.