And remember this -- use no apostrophes. Not even for initialisms.
Rare exceptions may be warranted, but this passage is plain wrong [emphases added]:
Sony BMG seems to have failed that test when, in seeking to limit consumers to making three copies of its CD's, it embedded the First 4 Internet software, which penetrates deeply into the PC's of users with a program that introduced a real, if minor, security risk.
Sad to say, but something like this doesn't really surprise me in some sources where I think a lack of competent editors might be an adequate excuse for such a mistake. But the passage quoted above appeared in the November 14, 2005 edition of The New York Times, "Business Day" section. This is a periodical that you might think would know better.
The editors really fell down on this one. Tom Zeller, Jr., the writer of the piece headlined "The Ghost in the CD", used the erroneous "CD's" at least nine times in his article, which clearly suggests that he believes this to be the correct way of indicating more than one CD, or compact disk. It is not. It is CDs, just as the plural of PC is PCs. And any editor at The New York Times should have caught that even on the most cursory reading of Mr. Zeller's article. That they didn't reflects poorly on the once proud paper.
Of course, I don't own a copy of The New York Times Manual of Style and Usage, so I guess it is possible that "CD's" and "PC's" are actually house style at the Gray Lady. If so, it looks sloppy, and it still reflects poorly on the paper.