Great ideas, thanks.

I did think of doing a geographic drill down. I figured if anything that would be the safest way to optimize for SEO without using any blackhat tricks like hidden links. That would give the users the option of searching via zip code or drilling through the site to their specific location.

I've never played around with mod-rewrite (even after 10 years of web development). But I use a similiar type of scripting which creates very much the same effect as mod-rewrite with PHP. My only problem or I guess question is would I include each zip code in the sitemap for the search engines? Not a big deal since that could be dynamically generated everytime we ad a listing to a zip code, just add that zip to the sitemap. To me though it seems like the search engines might question the pages since it would never land on a page featuring links to those zip pages. The search result pages obviously would be those zip pages, so the site map would essentially be a hardcoded link to those search results.

I actually like both ideas, maybe a combination of both would work the best?