A hint last year mentioned one way to speed up ripping, and there are others: turn off error correction (but risk getting diginoise in your music files), and quit extraneous applications to make sure that iTunes gets as much CPU time as possible.
But with a Mac Pro, another option is available: add a second optical drive. The built-in DVD burner reads CDs at up to 32x; this translates, on my Mac Pro, to an average of about 15x ripping in iTunes. So I bought a cheap CDR drive (not a DVD/CD drive) that reads CDs at up to 52x. The results are stunning: I now rip CDs at an average of over 30x, hitting over 40x at the end of the disc. (Ripping speeds are always slower at the beginning of a CD, because the CD itself turns at the same speed, but since there is less data per revolution near the middle, at the beginning of the CD, the drive simply reads less data.)
The only downside to this is the noise that the drive makes; but at less than 2 minutes per CD, it's more than acceptable.
And, before you ask, you cannot rip two CDs at the same time, at least not with iTunes. However, if you use an additional ripping program, you should probably be able to rip one with iTunes and the other with that program. I haven't tested that.

