The import GB Player does have a boot disc, so in order to use it you will need a Freeloader or Action Replay, unless you want to mod your system. All the menus will be in Japanese.
As for the disc, there have been a lot of people wondering if they could buy a replacement disc and not have to boot in Japanese each time. The other thing is, some people are wondering if the hardware is the exact same, and by using a different disc, it gives you the American menus. No one will really know until they get a US GB Player, unless someone can get Nintendo to talk about importing, which isn't likely.
You'll need to wait until PGC or IGN gets ahold of one, and then ask one of them to try for you. But if you're planning on importing, be prepared to be using the Japanese disc forever. I wouldn't hold your breath on a US start-up disc.
You could also wait closer to the US release. My GB Player cost me $83 with 3-day shipping. The US version costs $39.99. Prices might go down if you avoid the initial rush.