The whole discussion is actually pretty complex. If you want the whole story you'll probably need to find a bunch of back issues of Bicycle Quarterly. The French began using front racks in the 1930s and apparently learned, through decades of experience, what it takes to make it work.
The short version is that a 73 degree head angle with approximately 6 cm. of fork rake (that's not a typo) gives you the appropriate trail figure for stable-yet-light handling with a 5-10 pound front load. That's assuming 700c wheels and +/-25mm tires. Wider tires like less trail, which means more fork rake -- up to 7cm. As far as I know, it isn't possible to make a bike handle really nicely with a 15-20 pound load on a front rack, and low-mount front panniers may be a better option in that situation.
Last edited by Six jours; 09-28-08 at 11:03 AM.