I'm seriously thinking of buying a dualie. Most of the reasons have been mentioned, but there's one more reason why now is a good time to buy a desktop.
Indications are that Apple is going to join Dell in the Intel-only bed, at least for a while. Steve probably made the right decision for Steve: I'm sure Apple has some serious "marketing" dollars coming its way, and Intel mobile processors completely outclass the competition. But Intel desktop and server processors are completely outclassed by AMD and IBM processors. As pointed out in the anandtech article, the PowerMac G5 is very competitive with the x86 competition as long as you don't cripple it with OS X. All the money is in mobile, so you can't blame Steve, but it is annoying.
Don't Buy A Quad G5