USAGE: indefinite "a" before vowel-initial words

Idle question while I'm supposed to be writing a paper:

Does anyone know of any articles or research that's been done
on the distribution of the English indefinite article 'a' vs.
'an' when before vowel-initial words?  You'd think it's obvious
that "an" is always used in that environment, and indeed that's
the prescriptive distribution. But I've heard others using
"a" sometimes prevocalically (and without pausing), and have
noticed myself using it.  And now I just found while reading
about the newly discovered planetoid Sedna an example of the same:

"A alternative definition promoted by astronomers is that..."

(The context is such that it's unlikely that the "A" is being
used as some kind of bullet or organizational device:
<http://www.gps.caltech.edu/~mbrown/sedna/>)

So, it seems to be something more than my crazy "language
module" acting up again.  Maybe a sound change in progress?

 =========================================================================
Thomas Wier	       "I find it useful to meet my subjects personally,
Dept. of Linguistics    because our secret police don't get it right
University of Chicago   half the time." -- octogenarian Sheikh Zayed of
1010 E. 59th Street     Abu Dhabi, to a French reporter.
Chicago, IL 60637

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