Originally Posted by flargle

Here is the perfect bike for your needs and budget, but I fear you are too hung up on weight to go for it.

Gunnar Crosshairs frame and fork in 48cm or 50cm (whichever top tube most closely matches your road bike; you get to choose the paint scheme)
Dura-Ace gruppo (standard road gearing)
Mavic Ksyrium clinchers

If the Crosshairs doesn't cost enough, then get a custom Waterford cross frame. I believe their turnaround time is reasonable.

This bike will more than meet your commuting, light touring, fire-road riding, and cross racing needs. When cross season comes, buy a tubular wheelset (for example, Mavic Reflex laced to Dura-Ace hubs) for racing, and use the Ksyriums in the pit, and you will have a fast, reliable, and light racing setup.