An external hard drive, with a button on it that can launch a program in windows -- e.g. a backup or sync. I have only experienced using it with the USB port.
The button does not currently work anywhere but Windows, where it is supported by a program that periodically polls the drive for status. The polling happens by sending a SCSI INQUIRY command for vendor-specific "Vital Page Data" (VPD) 0xC3, which returns a pacaket of the form 00 C3 00 06 00 00 00 00 XX YY, with XX being the number of button-down events since the last poll, and YY being the current state of the button. The four 00 bytes may actually have some meaning, but if so, I have no idea what.
The original OneTouch had a much nicer interface from the point of view of the kernel -- it provided an additional USB endpoint that could tell the kernel "hey, I got pressed" without polling. My guess is that the addition of the Firewire port made this somehow less workable.
We can, however, easily replicate the windows behavior in user space. All we need to do is the same polling.