The possibility that the original iPhone will not be able to upgrade to iPhone OS 4 is not that big a deal - in the US, phones are basically on a 2-3 year cycle and so the fact that there is a user base of any size for a three year old phone is remarkable and a testament to the quality and cutting edge nature of the original iPhone.
The unusual problem is related to one of the iPhone OS's strengths - the iPod touch. Although less expensive, I would guess that the normal expected life is longer than that of a phone. For example, I'm still using (albeit in a sort of back-up, for the kids way) a relatively old iPod (it uses a firewire connector...). And, if the iPhone news is correct, it seems likely that at some point these touch users are no longer going to be able to upgrade their devices either. All of which would be fine without the other strength of the platform - the app store. As new apps start depending on the features of the newer OSes, these old devices will become less and less viable even though their owners may not think of them as expired. And, although there will be a giant universe of app choices, a "grass-is-greener" perspective will still make them feel shortchanged. I (irrationally) feel somewhat shortchanged by not being able to take advantage of apps that require a compass (only on the 3GS and not the 3G).
Finally, this will all get worse with the iPad which should have an even longer expected natural lifespan.
The price points for these devices are less than that of a computer (mostly) but still I'll analogize to Windows XP and its continued support and large userbase. I wonder if iPhone OSes two strengths might someday end up as (modest) handicaps. Having three strong devices plus the app store is a high class problem (Queen of England kind of class) but, as the platform continues to bring in users, I wonder what happens as normal device obsolescence kicks in for these (at one time) futuristic devices.
To be clear, there is no other phone that has a userbase and set of user expectations large enough to have an obsolescence issue. Phones were too rudimentary and people were so unattached that they got left behind pretty easily every two years. But the number of people using original iPhones as touches must be substantial.