You could go with a Scott Speedster frame... even the S6 frames ("lower end") are light.
The Speedster is the perfect race inspired bike featuring comfortable riding characteristics to endure the long distance rides. The 2008 Speedster series gets a shapely new hydro-formed 7005 butted alloy frameset sure to standout on the Sunday ride. Weighing a scant 2.62 lbs, the Speedster frame is lighter than many competitors' high end offerings. CR1 geometry, Addict carbon/alloy forks and quality components ensure maximum ride quality and long term durability in an economical package.
I myself have an S5, equipped with Xero XR-1's, CR1 fork, and sora/tiagra mix (triple). I'll be going full Ultegra soon. It's already light, but hopefully by the time I am done getting into race shape it will be very light (for an alu frame).