Hi TD Ann,
Personally I don't use sub domains at all for SEO reasons, only for practicality or usability. As has been pointed out the bulk of directories are human edited, and while they 'might' get through in different sections, there is a possibility that your competitors will report the fact, and the site gets reviewed, worse case scenario is that you get pulled under every instance, as lets be blunt, you ARE doing it purely for SEO reasons rather than a usability, or any other bona fide reason. That said, it is only a possibility that you could fall foul.
Also to be considered with Google is the fact that Page Rank flows better across a domain than it does across the web, so in fact any gains you get from splitting the site might well be lost in the dampening factor that is applied.
Thirdly As search is moving on, the use of 'word understanding' semantic technology is likely to get incorporated more and more, to give the search engines a better understanding of what a page, site, and relation between pages and sites is all about. Having 100 pages of good quality information is going to tell the SE's what your site is really about, and it is also going to encourage opportunity to secure good quality deep links. This PR will flow around the site freely, and might, end up being of more benefit than splitting the site into numerous sub domains.
Google certainly is going down the rout of Semantics, having bought Applied Semantics, and played with it during the Florida update of late 2003 (dear me I sound like an old timer gold prospector from way back when) Teoma uses clustering, and Google have also said that they use clustering using a 'hub' (a place that links out to many other related sites) and 'Authority' (a place where many related sites link into) system. This clusters sites around themed topics, and links from hubs, and authority sites are deemed to be more relevant than other links for on topic searches. If you split your site into pieces, then you run the risk of an authority link from one, not counting toward the rest of the pages on the other sub domains. Also, if you work on your site, build a resource, research your field, then 'your' site, could eventually become a hub, authority, or even both as is the case with many sites who link out, and have many links pointing back to them.
Hub & Authority sites are relative to the field where they operate in. If there are only 3 sites on the web about your subject (example) and you link to the other two, ant the other two both link to you, but not to each other, then YOUR site links to every other known page about your subject, and every other known page links to you, but every other known page does not link to each other, so that makes you both a hub, and an authority on your chosen subject.
Phew, I have gone off topic a little here (sorry) that’s how I am, get me talking and I can't shut up
Hope that has helped though
OWG
SEO pros/cons of sub-domains
