Well the program is pretty far underway so far, it's pretty trivial to write. Anyways, it creates an icon that sits in the tray, you click on it and it pops up a menu with all the commands, an exit command, and a setup command. Within setup, you'll be able to assign each command a hot key.

For volume up/volume down and FF/RW, since it would be annoying to only FF/RW for a split second, when you execute that command it will fast forward/rewind for about two seconds. Volume up/down, when clicking on the menu item, will turn up or lower the volume ten notches, while using the hot key will turn it up/down one notch (this is because it's easy to repeat-press the hot key rather than to keep clicking on the menu item).

The now and next idea is interesting, and I also thought a display of the current track playing as the tooltip would be nice, but I didn't see any info on the riocar.org site about receiving info via the serial, only sending commands.