acroynm

Well, technically this is not an acroynm or even a tag.  :)
 
"An acronym is defined as a WORD formed from the initial letters of a
multi-word name. The important point here is that an acronym must be a WORD
- this means that the joined initial letters must be able to be pronounced."
http://www.maxdesign.com.au/presentation/abbreviations/
 
Metropolitan Statistical Area (MSA) is actually an initialism - An
abbreviation pronounced as the names of the individual letters formed only
from the initial letter of constituent words.
 
Initialisms are subsets of abbreviations. So theoretically this should be
marked up using the <abbr> element:
<abbr title=" Metropolitan Statistical Area">M.S.A.</abbr>
 
The problem is that the <abbr> is poorly supported by IE5 and IE6. This
means you may have to (1) revert to using the <acronym> element, or (2)
place a span inside your <abbr> element and style this instead or (3) use
JavaScript:
http://annevankesteren.nl/2003/08/improved-styling-abbr-in-ie
 
The full stops between the letters in the initialism are used by some
authors in order to allow screen readers to spell out the letters properly
rather than coming out as a single unintelligible word - "msah".
 
Oh, and I'd vote for just the first instance on each page - as others have
suggested.
 
Thanks
Russ