I'm a little surprised that there hasn't been a mention of quilts, either, especially since BPL has been involved in their design and marketing for years. For the temps above freezing (32F, 0C) that have been talked about, a quilt is a no-brainer, and probably works for you in the widest range of temperatures. They're far more versatile than most mummy bags, and you can extend their bottom range with clothing far more easily, or even use them to extend the range of a mummy bag, while carrying very little weight for the amount of warmth they offer.
There are a wide variety of options here, including already-built down quilts like the Arc Alpinist series from Nunatak and those from JacksRBetter (www.jacksrbetter.com). And then there's the down quilt kit from www.thru-hiker.com. My Arc Alpinist X (now simply a quantum-shelled option at Nunatak) in LONG was around 20 ounces, but provides a LOT more warmth than something like the Marmot Pounder, which seems more like a 50 degree bag to me <g>.
There are also synthetic alternatives, and quilts with at least a couple of insulation alternatives are available from Fanatic Fringe (www.fanaticfringe.com). Ray Jardine, who really popularized quilts in Beyond Backpacking, has synthetic quilt kits available on his website (www.rayjardine.com). My first quilt was a synthetic quilt built to his instructions in the book, and it was simple to sew up and was my main sleeping "bag" for several years (the arc alpinist has relegated it to shelf status for the time being). I think that Backpackinglight.com may be offering synthetic insulation quilts in the near future -- Ryan's hinted at their arrival for months.
It's also worth noting that some design options have done much to make a simple quilt more appealing. One, of course, is the addition of a foot pocket. Another is the use of straps and snaps to snug the quilt around either the pad and the user or around the user himself, for increased warmth and draft protection in colder weather. Yet another is the use of draft flaps (see the Jardine site) which add little weight, but do much to get rid of the occasional cool "wakeup call." And finally, the addition of an extremely lightweight but breathable bivy not only subs for a ground cloth, but offers protection from spray and condensation while eliminating drafts. Makes a perfect adjunct for a quilt used under a tarp.