Mark,
Unless you replace the complete rod section, you will never get exactly the same action. However, if you are handy with epoxy, is is not difficult to repair. Here are a couple of links that will give you some ideas to work with:
http://flyanglersonline.com/features/rodrepair/
http://stevedobsonsfishermansblog.blogs ... p-top.html
As Todd mentioned, if it is near the tip you can get away with replacing the tip guide. I know that Float, Paddle and Fly has a variety of diameter tip guides.
If it is further down, a simple repair is to find another old or broken rod that is a larger diameter that the rod you broke. I've read that it is best to use a fiberglass rod to repair a graphite rod. You cut the fiberglass rod down so that it slides over the broken rod (first cut the broken end smooth) and then epoxy the two pieces together. Reportedly, although the "splice" adds stiffness, the fiberglass is usually more flexible than the graphite and compensates.
I have also read that if the rod is stiffer after the repair, it helps to use a heavier flyline to compensate (e.g., for a 6 wt rod try an 8 wt line).
Hope this helps.
Cheers,
Colin