Doh! I figured it out (sometimes less is more). Not surprisingly I suppose, the answer was in a tech-doc.
Apparently the confusion is due to terminology. What happens is that there are two causes for a system running in single-channel mode. In both cases, the memory does not use the multi-channel architecture it supports, but the specific cause of using a single-channel determines the actual label.
When only one slot of a channel has RAM in it and the other is empty, it is called “single-channel” mode. When both slots of a channel (or more in the case of triple- or quad-channel boards) have RAM in them, but they are not matched, that is called “virtual single-channel” mode (though many people just call it single-channel, thus causing all the confusion).
In other words (or pictures as the case may be), on a two-slot motherboard (for simplicity):
Single-Channel: [ 512MB ] [ ----- ] Virtual Single-Channel: [ 1GB ] [ 512MB ]