And another one that gets to the difficulty in answering the question (now
I'm running across things looking for something else). From "Writing
unwritten languages," Drafted by Clinton Robinson with Karl Gadelii,
December 2003 at
http://portal.unesco.org/education/en/ev.php-URL_ID=28301&URL_DO=DO_TOPIC&UR
L_SECTION=201.html
-or-
http://tinyurl.com/ykozlc

(in the context of the total number of languages in the world...)
"How many of these are written? It is extremely difficult to estimate how
many written and unwritten languages there are in the world, and there is no
established source of information. The difficulty in counting comes in part
from a lack of information of what is happening on the ground. The world
currently has no systematic way to collect data on the number of communities
which are developing their languages, what stage they have reached, whether
existing writing systems are actually used, or whether attempts have been
made to develop writing systems that are not in use. The Ethnologue notes
sporadically whether a language 'has an orthography' or 'has an official
orthography', but does not present information on writing systems for each
language."