With the price of gas, riding is cheaper.
At least that's what this article claims. They've calculated the total costs per year for a car at between $4-6,000, and a horse at about $1600 (which I think is a pretty low estimate, but they are Amish, so their expenses may be cheaper). I estimate horse costs at about $2,700/yr (I may write an article on this later).
I personally will be staying behind the wheel for my daily commute. It's about 80 miles round-trip to go to school 3 days a week. Riding a horse wouldn't be feasable.
But if, like some people I know, I lived close to campus, riding could definately save money-- especially with the current price of diesel, in my town, of $4.29/gallon (I'm stuck driving the old ranch truck for now).
One of the downsides I see to riding would be safety issues-- most places may have parking lots, but darn few have hitching posts or pens.