There was an article a week or so ago in the Detroit Free Press that mentioned something about "an NWA employee." As I was reading the article, I read "NWA" to myself as "Northwest Airlines" (out of habit), thus the preceding "an" sounded incorrect. Remember the rule - always use "a" before a word that begins with a consonant sound and "an" before a word that begins with a vowel sound. This got me to thinking about the use of the determiners "a" and "an" with acronyms and initialisms.
Acronyms are pretty much straightforward compared to initialisms. An acronym is generally formed from the initial letters or syllables of a name and it is always verbalized as a word - Think of NATO, it is never read or spoken of as "N"-"A"-"T"-"O" so there is not a question that it would be "a NATO meeting," and not "an 'N'-'A'-'T'-'O' meeting."
On the other hand, initialisms, though also formed from the initial letters of a name and written as such, are not always verbalized as individual letters (contrary to the definition). This is why when reading "an NWA employee" as "an Northwest Airlines employee," the use of "an" as a determiner sounded and would be considered incorrect.
I could not find a rule regarding this problem in the Associated Press Stylebook. I would guess that because initialisms are made of initials and the initials are not used to form a separate word as they are in acronyms, determiner usage should be based on the verbalization of the first letter of the initialism.
In sum, I believe the Free Press was correct and it was my fault for reading "NWA" as "Northwest Airlines." The only problem would be if a person encountered an unknown abbreviation that could be read as an acronym or an initialism.