I would say you need to have crawlable links from the initial landing page. Can you not create a page with links to the different sites rather than having a drop down box? You could then have text links for the search engines to crawl. Alternatively you could use a javascript drop down.
As long as the search engines can get past the initial page there should be no issues with them crawling the rest of the site.
I would also question how you are going to display the different versions of the site in terms of duplicate content. Are the different sites going to be hosted in different subfolders? e.g. www.domain.com/en and www.domain.com/us?
If they are going to be hosted like this I would strongly recommend considering your strategy for avoiding duplicate content across these sites.
