Hi Dave,
Yeah, this would probably hurt you. I am a little curious why you would want to do this in the first place.
If you keep your site and subcategories centralized around one theme anyhow, this would help you in the search engines, not hinder you. Just make sure you avoid theme bleeding as much as possible.
If your trying to just save yourself a few bucks on different domain names, well it isn't worth it. Leave your overall theme of your site in tact!
With the advent of LSI, the search engine spiders will probe your site for like terms, and or, sysnonyms, which may be present around a paricular theme. While this is a great thing for building sites using a silo structure, it is also a very good reason to keep your overall theme, and subthemes, tightly focused.
The reason this would not work as far as when you are submitting your site to the online directories is the most, probably 95% of them only allow top level domains to be submitted.
I have a lot of information on this type of thing on my site
http://www.davinatorbiz.com , if you would like to check that out.
Cheers
Davin
Post edited by: The Davinator, at: 2007/07/11 00:26