Same here. Rockbox 3.9.1 on iPod nano 1st gen. Happens (or fails to happen) consistently on Linux (Gentoo). I even tried it on a Mac.
Symptom is: when I plug in via USB, screen changes to a USB cable. When I type 'eject /dev/ipod' the ipod screen remains as-is (USB cable). Same thing on the Mac when pressing the Eject arrow. (I expected the device to recognize that it has been unmounted, and to display a different screen).
I'm new to Rockbox. I've searched web & forums for eject, eject+nano, dismount, various other combinations. Searched the bug database.