^ Confirmed.
SPDIF via RCA cable. Display on the 680 will indicate DIN LOCK when signal is detected from the V3 and will lock to the SPDIF clock signal. Pushing the ENTER key to dismisses the notice. Similarly, the display will indicate DIN UNLOCK if the connection is lost, upon which the 680 will revert to it's internal 680 clock, and does so seamlessly without glitching the recording.
So I'm apparently doing something wrong. I'm trying to test this before a couple of shows starting tomorrow night, and I'm not getting any notice from the 680 that it's seeing the signal from the V3. I've got the STEREO REC set to DIN to record the digital input to tracks 7/8. The V3 is on and metering properly when I clap my hands, but nothing is registering on the 680.
EDIT: So I changed the settings to record the digital input to channels 5/6 and suddenly I'm seeing the levels on the 680, but I haven't seen the message telling me that it's locked to the V3. Is this a problem?
EDIT 2: I figured it out (at least with respect to channels 5/6). With the 680 and V3 powered on, it didn't give me any message about seeing the V3 when I changed the settings in the 680 REC and I/O menus, but when I powered off the 680 and turned it back on, the message popped up. It also pops up if I turn the V3 on or off.