Clones are good for the consumer, but bad for Apple.
They're good for the consumer, because you now have more choices. You can stick with Apple gear, you can get a dirt-cheap ugly box that just gets the job done (like Dell's stuff), or you can get a souped-up hot-rod system (like Alienware).
They're bad for Apple because lots of people who today are buying Apple equipment will choose to buy a clone. If too many do (as happened last time around), it can even kill the company. Clones can only help Apple if it serves to greatly incrase the Mac OS user-base. If it doesn't, but simply moves Mac users to other hardware vendors, then it's counterproductive.
WRT some of the arguments others have posted here...
Apple will not become another Dell. Maybe there will be cheap Dell-like companies selling Mac clones, but that won't affect the quality or support Apple will give to people buying Apple equipment.
Sure, some of the clones will be ugly. So what? The people who want looks can continue to buy Apple gear. Those that don't care about looks can bey the clone. Wanting to pay a premium for Ive designs shouldn't be a prerequisite for wanting to use Mac OS.
As for the mini, it is not the only possible low-cost system, you know. A lot of people would like to have a low-cost system with PCI slots, like you find in many cheap PC's. A stereotypical minitower may be ugly, but lots of people would be more than willing to sacrifice looks in favor of internal expansion capability. You shouldn't be forced to buy a PowerMac (at least four times the price) in order to get this.
What everyone seems to forget is that if Apple starts selling system software that can run on generic PC's, they will be going head-to-head against Microsoft on their home turf. The market is littered with the remains of products that tried this (OS/2, BeOS, OpenSTEP, etc.) I think Apple would be incredibly stupid to try this tactic. Better to wait for Microsoft to implode on its own (which it will, eventually, but possibly not for another 5-10 years) and then see about selling into that market.