hey
that's all very cool. I will post my files in the next days; they're licensed CC-BY-SA, so you can manufacture them if you want. Eventually I will post the gerbers, parts list and supplier info too. All open 
My design is stacking, so you can put as many shields as you want on top of each other... theoretically you could have a stack of eight motor shields without any problems.
Sorry I don't have the time to help you with your design, but maybe these files will give you some ideas!
Another price item you might want to check out are the Schottky diodes: you'll need 24 of them, and they are not cheap: count on 50 cents each in small quantities. You have to buy in quantity 500 or 100 to get a good price.
I think you could do the Atmega instead of L297 route quite easily, using the Analog in pins (2 for each motor) to watch the motor current and calculate the chopping. In fact it seems like the Atmega 168 has pretty much the same functions as the L297, except that it's programmable and the code can be open-sourced if you go that route.
D
PS: BTW I am not one of the folks "at Arduino".... I guess you haven't read the threads under the news section!
I'm one of the folks at Freeduino 