Apple already said that iOS 5 will be on the 3GS.
To be fair, when this discussion was posted on June 1st (see date above), iOS 5 hadn't been announced yet.
It might, but it will probably have limited functionality compared to later generation phones (see what happened with the 3G running iOS 4). Who knows what new implementations iOS 5 will add, but some of the cooler things rumored (e.g., widgets and notifications) will most likely not make an appearance on the iPhone 3GS.
It's a bummer, but it's the march of technology. Apple is also probably betting on the fact that it's a two year old phone (let's be honest, that's AGES in the mobile world), so whoever has it will be due for an upgrade soon anyway.
Yeah, you're definitely right. The 3GS could potentially not be able to handle the new widgets and notifications just as the 3G can't handle multitasking.
Either way, 3GS owners will be able to upgrade if they bought it when it first came out. Nowadays, it certainly seems as if smartphones are only supposed to last up to 2 years at max. So in a sense Apple is correct in providing 2 updates. The update on the second year is essential and the update 2 iPhones later is for those who will only be using it for part of the year before they upgrade.
Everyone seems to be taking what happened with iOS 4 as the norm and applying it to iOS 5. Thing is, it probably won't. The reason that iOS 4 wasn't 100% supported by the 3G was due to performance. I would imagine that the 3GS will be more than capable of running iOS 5 with all features. Unless they do something crazy.
It will probably be provided, but some of the features may not work as well or not be available at all. Seems to be Apple's MO for such upgrades.
Seriously. For example, you don't see Android 2.2 or 2.3 officially supported on an HTC Hero (released in late 2009 -- after the iPhone 3GS) or other, older phones. There's a reason why.