You could use client-side solutions too. Try to use EMIGR (emigr.com). That's simple and useful tool.

What do you want to serve that would be different? Are we talking language localization, sales or product info or terms and conditions, etc.?

I'd use the ability to serve different content, but only to point to a page with that specific content, so that both versions can be indexed properly, and you can control which page has content that gets indexed and all that...

You can use PHP GeoIP functions in order to analyze and determine your user's location dependant on their IP. You'd then be able to serve the corresponding content based on this information.

Check out GeoIP.