I am thinking of getting a MacPro notebook when I visit the US. How can I use the US-bought Mac to process Japanese? How do I input Japanese using the US keyboard?
You simply enable the dialect of Japanese that you want within System Preferences >> Language & Text. Then you may select the language you want to use on the fly from a menu bar. You may or may not need a Japanese keyboard depending on your preferences. From my understanding, may Japanese speakers will use a Roman character keyboard and type out the words phonetically. Japanese keyboard have a few extra characters and a layout that is more comfortable for those that are accustomed to it, however you can get by nicely (and others do) with the Latin alphabet that is on the US keyboard.