I used my REI dividend and 20% off coupon today to get a Kelty Cosmic Down 20 19*F rated bag (REI link to bag. Amazon link to bag.) It weighs 2 lbs. 9 oz., and is filled with 600 fill moisture-resistant DriDown. It came with a stuff sack but no storage bag so I also bought an REI-brand cotton bag to store it in uncompressed.
If I'm expecting colder weather I can use my HPG Mountain Serape as an overbag. The combination should get me down into the teens and still be a lot lighter than my USGI Military Sleep System.
While at REI I also grabbed a couple packs of Nite Ize Gear Ties, which are very handy for organizing cables that go with electronics. Between my dividend and the coupon, I was only out about $50.
Once I get the chance to use the Kelty bag I'll post a report, but it may be awhile.