I have a PC version of the USB Kinesis Freestyle keyboard. It've used these keyboards for years with my Macs and they work very well for me, but I have never used the block of keys along the left-hand side.
The back/fwd keys in particular would be ripe for use to control iTunes, but the problem is that pressing them generates ⌘-arrow key sequences, and remapping ⌘-arrow sequences would cause problems in applications that use ⌘-arrow keys for editing functions.
Is there a way to change the character sequence that the Kinesis keyboard generates?