After the revolutionary1 deal of brazen product placement in Geass, few people even mentioned the logo of Yamaha in Nodame.
I imagine it makes the licensing more difficult, although it did not stop Geass from being imported.
UPDATE: Look at an excellent example at Karmaburn: Haagen-Dazs in Kara no Kyokai.
1. Check out how Jason Miao was surprised, for instance.