Oh limewave, how disappointing! My opinion is to keep on running so long as you can withstand the pain. If it's not going to do any more damage, then I'd go for it. Think of how much studlier you'll be after running a marathon on an injured foot! Wow! Can they give you a cortisone shot to relieve some of the pain (don't know if that's the right drug I'm talking about)?
Similar thing happened to me last year. 6 weeks out from a marathon (my 2nd), I injured my knee to the point where I could not run at all. The pain was terrible. When I walked, tho, it didn't hurt so bad. My doc told me to stop running, so I did. I was determined to finish the marathon tho, after training for 4 months, so I walked it instead. It wasn't fast, but I actually had a great time. I got to high-five all the kids, pet the dogs, and enjoy the experience (and since it took me 7 hours- there was lot's to enjoy ).
If you don't mind walking, and can combine that with the running, I would still do it. The Chicago Marathon is extraordinary, with so many spectators and entertainment. If you have to walk part of the way, you know you'll have lots to look at and lots of people cheering you on.
Best of luck to you and your foot! Let us know how it all turns out!