I found this version of a line-following robot in a book a while back. It uses photoresistors rather than infrared phototransistors, but I think they should work about the same. Please correct me if i'm wrong (anybody), but I believe you can simply replace the photoresistors with the infrared detectors, and replace the "headlights" with your infrared LEDs. Here's the link to the website:
I'm not sure if he has the actual circuit on the site or not, but if you cant find it, I recommend you see if you can get a hold of the book itself.
I hope this helps!
Der Strom