Seems like an odd place to ask such a question but I'm not complaining. I'm surprised that you uploaded the schematic right away, on the electronics forum I frequent, they usually seem to forget that important detail. When I was in high school I found a blog about someone who tinkered with a similar robot design, I'll see if I can dig that up and post it here. As for your design I have some constructive criticism for you.
The LM324 has minimal output power, you are going to want a current boosting resistor in order to drive the motor. I would suggest using an NPN on the negative end of your motor since it will only have a voltage drop of 0.7V. I would also suggest adding a fly-back diode to the motor since there will be a considerable amount of energy stored in it's coils.
As a line follower having a single sensor isn't going to work (I'm assuming you know this since the 324 is a quad op-amp). Have a sensor on each side of the line and have them look for the floor. The presence of the line will shut off the motor that is controlled by that sensor and the robot will adjust itself. You can also use photo diodes as the sensor since they respond much more quickly to changes in light than the LDRs will.
As for using the IR LED/ transistor combo, I see no reason why that couldn't work. I would think that it will have some trouble with non-reflective objects however. You can have the output trick the motor controllers into thinking that they've seen the line at the same time which would bring the robot to a halt.
I can rummage around through some of my data sheets and see if I can dig up a good medium power transistor.
