I've just looked at around two dozen online guides to style and grammar, and every single one so far has said that "CD's" and "CDs" are both acceptable. Apostrophes, many say, have three uses: Possessive, contraction, and (optionally) in the plurals of acronyms and abbreviations. The New York Times apparently consistently uses "CD's", though most other newspapers have opted to go the "CD's" route. Personally, I'd use CDs, but when the acronym ends with a vowel, I would dither about adding that apostrophe for clarity. "CD's" and "CDs" are at least pronounced the same, but see "BAs" and most people would probably mentally read "bas".