For spring, fall and higher altitude hiking light down jackets ("sweaters") seem to be the answer to light, warm insulation. I recently had a wake-up experience on the southern PCT near Olancha Peak.
On two mornings in a high valley (8,000 ft) we had temps of 28 F. and 24 F. All I had was a 200 weight fleece vest under my PacLite rain parka, which was barely enough. On the last day out from that camp if got down to 16 F. !! and THAT experience promped me to buy an Eddie Bauer down sweater this summer. Got it on sale for $87.!!
To me a down sweater is becoming as basic a part of lightweight backpacking equipment as a good down bag. Treating them with a good DWR like Revivex or Techtron is important in keeping them dry, as is a light roll-top waterproof stuff sack.
Eric