In two recent articles Nobuo Uematsu commented on his involvement in Square Enix's biggest title for the current fiscal year, Final Fantasy XII. On May 29th, Uematsu said he was thinking about some ideas he could use in Final Fantasy XII. While he hadn't looked into it, he was looking forward to score the newest installment of the series. One week later, on June 6th, he mentioned that a Final Fantasy XII conference had been held and he was getting excited when thinking about starting to work on the game.
Considering Square Enix's plans to release Final Fantasy XII in Japan until the end of the current fiscal year (meaning March 2004 at the latest) it is pretty reasonable to assume that work on the soundtrack has begun by now.