Last winter the folks at Swiftwick were kind enough to give me a pair of their Ole Arm Warmers ($22.50) . I have extensively used and previously reviewed their light compression wool blend Merino 12 (12") socks in all seasons with great results.
At first I didn't see the purpose of arm warmers beyond the flexibility of rolling them down as temps and pace warmed during a run. Since then I have worn the Ole on warm and cool days and find them highly effective over a wide range of temperatures. I almost never roll them down.
Tightly knit and constructed in part of Olefin a highly hydrophobic fiber I find they manage temperatures well over a wide range. They never seem to get soaked with sweat and thus stay warm. When its warm the moisture seems to have a slight cooling effect and the long sleeves protect from the sun. I find that the gap between the top of the sleeves and a t-shirt contributes to helping regulate temperatures. They are quite windproof. Most will find sizing up a bit more comfortable. I got the large.
What to wear on a breezy day in the 40's? Combine Ole Arm Warmers with a merino wool t-shirt.