Our friends at Play-Asia have received the Game Boy Player and put it through some tests.
The Game Boy Player was just released in Japan, and importers will be happy to know that the peripheral is fully compatible with modded U.S. GameCubes. Also, U.S. GameCubes using the FreeLoader work fine. Additionally, PAL GameCubes can use the Japanese Game Boy Player with the FreeLoader and RGB cables (you need these for color display output). Finally, as one would expect, the Japanese GB Player does not work with unmodded U.S. GameCubes that are not using the FreeLoader.
So, for those of you who can't wait for the GB Player to be released in your territory, or if you want a non-black GB Player, import away! Thanks to Play-Asia for the info!