I suppose there could be some discussion as to what qualifies as an acronym, and what is actually an abbreviation. (Personally, I think acronym is often used when abbr is more appropriate.) Even Webster's seems to be unclear. For example:
A word formed from the initial letters of a name, such as WAC for Women's Army Corps, or by combining initial letters or parts of a series of words, such as radar for radio detecting and ranging.
n : a word formed from the initial letters of a multi-word name
1. The act or product of shortening.
2. A shortened form of a word or phrase used chiefly in writing to represent the complete form, such as Mass. for Massachusetts or USMC for United States Marine Corps.
3.The form to which a word or phrase is reduced by contraction and omission; a letter or letters, standing for a word or phrase of which they are a part; as, Gen. for Genesis; U.S.A. for United States of America.
So, both USMC and U.S.A. are used as examples of an abbreviation, but also seem to meet the definition of an acronym (a word formed from the initial letters of a name.) I would use the abbr tag for these, since you don't often refer to the Marines as the You Ess Em See, but you do refer to the Women's Army Corps as the Whack. Likewise, you don't often say You Ess Ay...just You Ess, or America. I think the key to acronym is that it is a word (WAC = Whack, not Double-U Ay See.)
Here's an article by Craig Saila, arguing that darn near everything we currently wrap in an acronym tag actually (correctly) belongs in an abbr tag.
I really liked one of the comments from Craig's article:
* : Use only with strict definition, i.e. for pronounceable abbreviations—if at all.
* : Use if: not pronounceable, always, or when in doubt.