I've written a HOWTO on how to use an iRiver media player together with Linux for TUX Magazine.
It includes both ways: using specialized software and leaving the original firmware as it is, and changing the firmware to make iRiver behave like an UMS device.
So, go get this issue (for free) and while you're at it, why don't you subscribe (for free) as well.
Update: TUX Magazine has stopped, but the website remains intact.