Keyboards will contain numerous extra keys and the Japanese character markings that you won't need. Warranty will also be difficult because 1. the warranty could be non-Worldwide and 2. even if it's worldwide, the regional service centre may not have the same parts, resulting in huge delays to any repairs. Also, you'd have to reinstall the OS and it might be difficult to find the exact drivers for the built-in peripherals if the equivalent model is not available outside of Japan.

Unless you have a particular requirement that can only be satisfied by a Japanese laptop, I would suggest you stick to buying from inside the USA. Tiny laptops are available in USA too - a couple of VAIO models (quite pricey), and the IBM X40/X60 for example (IBM X-series uses external optical drives, though).