I'm using a Linksys wireless router which I bought in Japan... I am almost certain it has exactly the same spec as the one that I saw at a colleague's home in the USA. (Well - It looks the same and it has the same product code...)

I believe the Japanese government requires an importer (such as a Japanese company which is distributing Linksys products here) to get a license for the wireless products that they want to sell in Japan. Some friends of mine had to do that in order to start selling a Chinese-made radio product here - they did not have to make any changes to the product - just fill in a lot of forms to show that it met Japanese standards.

If you plan to start an import business for electronics, you'd better check the law. But, frankly, nobody is going to stop you if you are carrying ONE wireless router in your suitcase. There are surely dozens of people in Okinawa who are using wireless routers which were purchased in the US.