It seems ignorant to say that a cheap headless Mac will kill sales of the G5 iMac. The reson I say this is because the eMac is already in the market as a cheap alternative. If the eMac didn't kill iMac sales, then why would a cheap machine without a monitor kill it? People buy the G5 iMac for its power, low footprint, and sheer elegance. The people who want a cheap Mac to do the basic essentials either buy a used machine, an eMac, or an iBook. The iMac is not the cheap apple alternative that it used to be in the G3 days. It has become a symbol of sleek apple engineering, and considerable computing power. That's the very reason they introduced the eMac, to cover the low end consumer market that the iMac sould no longer cover. At this point in the game, while the eMac is a good cheap alternative, Apple would do well to draw some more of the PC world into the fold, and the best way to do this is via a cheap headless machine. I know that I have personally lost sales of iMacs and eMacs because of the statement "but I already have a monitor". Unfortunately, many people are willing to toss away a $1000 tower, but demand to keep using a $100-200 monitor.
As for the complaints about Apple gaining too much market share, it is not such a bad thing for this to occur. If Apple gets more business, it makes more money. If it makes more money, we get to see more cool products. What's the problem with that? If you want Apple to keep its low market share just because it will make you feel special, then you hardly have an argument. There's nothing wrong with a large Apple marketshare. In fact, a greater marketshare would allow for more development in the Mac field. The only potential downside would be the development of viruses and spyware. However, if this were to become a problem, I have little doubt that Apple would be able to handle it in a timely and sensible manner (unlike the garbage that is Windows XP SP2). More and more people would see how wonderful OS X is, and get to own pretty computers. There's not really a problem here.
A new headless Mac would not drop the resale price of older PowerMacs too significantly. The eMac has had little effect on the sale of older PowerMacs. Really, nor have the iMacs. This machine is not a PowerMac. It is effectively an iMac or eMac. So, why should this machine have a substantial effect on the used market when its predecessors have not? People pay a premium for expandability in the Mac field, because those are the ones who truly need it. The number of people willing to buy a used PowerMac may decrease, but not the price it would sell for.
As for the claims that "this must be some sort of specialty device", that just seems ludicrous. The rumours about set top boxes, and other specialized Apple devices have been floating around for years because they "fit into the Apple picture". They may fit into the picture, but they only satisfy niche markets. Apple doesn't want another G4 Cube on their hands. That is, they don't want to be sitting on an overpriced machine that is only appealing to a niche market, when they could have made one that appealed more broadly. Apple has been in just the situation that many suggest it is getting itself into now, and had a heap of troubles as a result. Why would they do it again? A cheap headless Mac would be a good addition to their product line, and would attract a number of switchers. I can't see any problem with this whatsoever.