> The grammar rules I was taught are quite clear that the spelling and
> pronunciation of the acronym's expansion, rather than the spelling and
> pronunciation of the acronym itself, are what affect grammar.

I hesitate to continue this thread, but I've never heard of such a
rule, nor have I heard anyone say "a" rather than "an" before such
an acronym.  The reason we have "an" is that it sounds better, so it
makes sense to write it the way we say it.  

> Hence, I always say `a NFS server', or `a MMU', or whatever.

If you really mean that you *say* that, I think you're probably in
a tiny minority.

The first dictionary that comes to hand (the Concise Oxford) gives
"an MP" as an example of the use of "an".

-- Richard