Offhand, the only time I've seen apostrophes used for plurals is with letters and numbers as in "Mind your p's and q's," "Cross your t's and dot your i's," and "Cross out all 2's in the first row." CDs looks preferable to me as does cds, but because of the p's and q's, I can see how CD's came to be written. Again, it's just a matter of training your eye for whatever convention is generally accepted.