According to the N-Sider, yes Sega does have Dev kits for the GameCube. According to N-Sider, Sega has moved passed preliminary negotiations with Microsoft and Nintendo. They are apparently getting full cooperation from both industry giants. Sega has already ordered dev kits for both systems. N-Sider has this to say:

Sega's teams could already be working on some preliminary GameCube software. It should be very interesting what comes of this, particularly from Yuji Naka's Sonic Team, who already have two Game Boy Advance projects on their plate.