[cgray4]

It is fairly common to pluralize period-less abbreviations with apostrophes. The New York Times, for example, follows that standard. It is more common still for abbreviations with periods to take an apostrophe.

The New York Times doesn't do a great job of following that standard if indeed they try to. Searching for CDs gives me many more examples of "CDs" than "CD's" -- the only one I saw was a blog post, which is probably copy-edited less strictly than an article in the paper. Similarly, searching for ATMs gives "ATMs" and "A.T.M.'s", but not "ATM's". Others I found: both "GTOs" and "GTO's", "LCDs", "TVs", both "B&Bs" and "B&B's". The phrase "A's and B's", referring to grades, was always written with apostrophes, as was the phrase "P's and Q's".

One of the fun thing of doing these searches is that really old articles are mixed in with the results. Searching for LPs shows that it has been written "LPs" since at least 1955 and also yields this intriguing headline: "DIGITAL COMPACT DISKS - REPLACEMENT FOR LPs?" from 1983.