Sorry, I was mistaken. Speedwell2's examples, posted above on June 17, 2004, 12:35pm, are correct. I confused "acronyms" with "abbreviations."

Don't use an apostrophe for plural abbreviations that are more than 1 letter, whether in the UK or the USA (or anywhere else), unless the abbreviation has its own punctuation.

Examples:
We met 3 VIPs this year.
We hooded 12 Ph.D.'s at the most recent commencement ceremony.
I handed out IOUs to everyone at the race.

If the abbreviation is really an acronym, however, then you should only add a lowercase s to form the plural:

If we had more UNICEFs in the world, we might have less hunger.

What makes it an acronym? I think it's this: if one pronounces the letters, one after another, when it's spoken aloud, then it's an abbreviation. Example: we say, "In the UK" but it *sounds like* "in the you kay" because we are pronouncing each letter.

If one pronounces the acronym as if it were a word, then it's an acronym. Example: we say, "Four KARTS and thirteen NASCARs after his first race, Hollis McGumby was the winningest driver in the nation." Another example: If we had two NASDAQs, things would be different."

It's never correct to use an apostrophe for plural acronyms, whether in the UK or the USA (or anywhere else).