Interval Subtraction From the Extended Exercise beginning in section 2.1.4, you can find exercise 2.8:


  Exercise 2.8.  Using reasoning
  analogous to Alyssa's, describe how
  the difference of two intervals may be
  computed. Define a corresponding
  subtraction procedure, called
  sub-interval.


I wrote the following:

(define (sub-interval x y)
  (make-interval (- (lower-bound x) (upper-bound y))
                 (- (upper-bound x) (lower-bound y))))


What do you think?
Looks great.  Don't forget to define lower-bound and upper-bound as car and cdr respectively.