Question re: intro page to select region

Our site has a common framework, but loads different content (advertisers etc.) based on the region the user selects. Each region is a different subdomain. Currently when a user goes in they go to a "default" region that shows common data. We want to change the functionality so that when a new user comes to the site (when a cookie is not detected) the user is taken to a splash screen that asks them to select a region, then takes them to that region specific part of the site (regionname.domain.com) and sets a cookie so next time they will go directly to that region.

What I am worried about is search engines seeing the region select screen instead of the main index page. Also, thinking the search engines might devalue the pages since we will have similar versions of each page listed for 25 or so subdomains.

Any ideas on what would be a good way to force the search engine to bypass the region select intro screen? Or to ensure the search engines index the site as a whole with more weight than each subdomain? Any suggestions/insite would be appreciated.