I just had a bike modified for me at my LBS. Amongst all the other things they did was to add 2 holes to my forks for a front rack mount (The forks already had the 2 outside holes, I just wanted the inside hole put on so I could use my Blackburn Lowrider front mount).
The holes are the same as the water bottle holes where the holder mounts into your frame. They drilled a hole in the fork, and brazed in a threaded insert. My LBS charges $15 a hole, so it was $30 to have the two holes added. Now the brazing did burn off the paint of my black forks, but they did a quick respray with shiny black automotive rust paint and it looks great.
The other style of mid-fork attatchment is a threaded boss that is brazed to the front edge of the fork. No drilling required and you can thread from both sides to accomodate separate low-rider style racks*.
Tubus makes clamp-on adapter sets to fit their Tara front rack for bikes without threaded braze-ons on the forks. Here's a link. Follow the adapter link part way down the page to see what I mean. BTW, the Tara rack is a nice solid rack.