If computer history has taught us anything, it is that all computers will eventually be upgradeable IF there is money to be made at it, and IF it is technically feasible. So far, all Macs have pretty much been upgradeable and I don't see why clever and well-funded companies could not figure out how to keep on making upgrades. However, it will be a while before you will see an upgrade for a G5 Mac. Why? The Macs still use the fastest G5 processors, so what benefit would an upgrade bring? (Granted, you could go from a single 1.6 to a dual 2.7, but the additional hardware needed to cool and house the processors, not to mention the difficulties of memory, bus speed, etc. etc. make this at present nearly impossible and way too costly) In the future, they will surely come out with much faster processors that will be able to be shrunk and cooled sufficient to put in a present day G5. There is no doubt in my mind that it will happen one day. They will eventually figure out new ways to shape processors such that they could fit in almost any computer (for example, perhaps a future processor would be VERY flat and large, say 6 inches square or bigger (to help dissipate heat better), liquid and/or chemically cooled via some internally powered process, and much more powerful because of nano-technology or some other means), and perhaps with a different or non-existent heat sink system (because of advanced cooling techniques (why have they not thought to put a traditional air conditioning type unit in a computer case where the processor sits and seal it really well to avoid damage to the motherboard? is it possible? or is it really inefficient compared to the cooling of the present day G5?)). Point is, while I don't know how it will happen, I am sure that it WILL happen. I also wonder whether it will be worth it. Processor upgrades are making less and less sense as the price of new macs continue to go down, and as the connection protocols (USB, firewire, etc) relatively stay the same over longer periods of time (a common excuse to want to get a processor upgrade is that you already invested a bunch in peripherals that would not work with a new computer--this is becoming less the case as Firewire, bluetooth, USB 2 and PCI-X or Extreme become longer term standards--your G5 may not seem very upgradable if you could just pay a little more for a new mac versus a processor upgrade (especially if you have to get a better video card to more closely match the new mac's native card) and continue to use all your peripherals.
The REAL cost savings is in doing what I do because I am too poor to live on the edge, and that is to ride the "just becoming worthless" wave of used Macs and then upgrade them for relatively cheap and still having a useful machine. Until a year ago I used a 7100 upgraded with a G3/500 card. It was useable and screamed in OS 9, but was not upgradeable to OS X and did not support IDE or PCI and therefore needed to be put out to pasture. So I got a 7300 for almost nothing, then a G4/450 card for it and it was a step up performance wise. Again, it screamed in OS 9. But now (thanks to XPostFacto) I could see what all the OS X fuss was about. WOW. OS X really is great in many ways. So I got 544mb ram for the old bessie and a PCI card to run IDE hard drives and I was rockin. It ran Panther really well, BUT there was a big negative, and that was the video card (2 mb of VRAM). It ran most all mpeg or mov files with no stuttering, but for .wmv or .avi files--forget about it--it was stutter city. A month or so ago, I finally decided I needed something better and found that the Blue and White G3 machines were finally cheap enough to make them worthwhile. I paid $51 for mine, then got the ram up to 768mb for under $50, transferred all the IDE hard drives over (along the with PCI controller card), brought the external SCSI CD burner along for the ride thanks to the PCI SCSI card, and still managed to keep using all my old peripherals such as my serial port based Midi interface thanks to a serial port adapter for the B & W G3. I also found a G4/450 processor upgrade for $60 or so. So, for less than $200, I have a pretty peppy computer for most tasks. .wmv files play fine now, as do .avi's (unless they are really large picture movies). I can play DVD movies on it with only occasional small stutters at full screen. The only slow downs occur when doing major processor crunching tasks--burning CDs, or copying huge files, or trying to run many tracks in Garageband or Digital Performer. It is perfect for web surfing, but not really up to snuff as a music making machine. It is good for midi, but not as good for audio.
Nonetheless, this is place where many mac users have to live until their fortunes improve. Thus, I think companies will continue to find a way to make processor upgrades in the future.
