Well, I have a couple of thoughts on this... first off, it's about time. People are selling used Macs for WAY too much. I see some of the MDD PMs going for close to what a G5 refurb PM goes for! That's crazy, especially considerig the refurb gets the standard warranty, as is eligible for Applecare.
If there is a headless mini-mac it will certainly pring the prices down on G4 iMacs, and single CPU G4 powermacs some. Dual G4 PMs will still be grossly overpriced, I'm sure. On the other had, as mentioned by the previous poster, the G4 is upgradable with new video cards, etc, so they will still be valuable.
My other thought is that the new iBooks, which is just as fast, if not faster, than a lot of the older PBs, but they (the PBs) still sell for way more than they are, imo, worth. I was looking at getting a used PM G4, either a fast single CPU or an older dual G4. After searching around for awhile, I decided that an iMac G5 was a better buy. About the same price, all said and done, and more 'future proof'.
I'm not sure what it will finally take for the used Mac market to come down to reality, and hopefully this mini-mac will help, but we'll see.