New Website Design Considerations

I am in the design phase of getting a website up and running that will have scalability as a main concern.  I am looking for opinions on architecture and the like for this endeavor.

The site has a few unique characteristics that make scalability difficult.  Users will all have a pretty large amount of data that other users will be able to search.  The site will be entirely based around search.  The catch is that other users will be searching always with a stipulation of 'n' miles from me.  I imagine that fact will kill the possibility of query caching for most searches.

I have extensive experience with PHP and MYSQL, some experience with ASP.NET/C#, some experience with perl but can learn anything fast.  The site will start out on a single server but I want to be 100% certain that I architect the code and databases such that scaling will be simple.

What language should I code the site in?  What DB would you use: Postgres, MYSQL, MSSQL, BerkelyDB?  Should we shard the database by location?  by user?  not at all?

What does everyone think for possible architectures on this?