I like the mentions of "A4 processor" over and over.
A4 is the name given to the single CPU+GPU combo. Multiple chips on a single die, not a processor.
The iPhone 3GS and iPhone 4 have the same CPU and GPU. The iPhone 4's CPU is clocked a bit higher, though (but it also has to power double the video resolution).
The big advantage with the iPhone 4 comes from its extra RAM. I could see a new OS requiring more RAM than what the 3GS has.
However, the iPhone 3GS has the same amount of RAM as the 4th iPod Touch and the iPad. If Apple prevents iOS 5 on the 3GS because of RAM, wouldn't it have to block it on the 4th iPod Touch and iPad as well then?
The 3GS is too close to current hardware. Many people like to compare it to the old 3G because they visually look the same, even though the guts of the 3GS are more similar to the iPhone 4.
Also, all previous devices have received two iOS updates. They are still selling (a lot) of new 3GS phones now with iOS 4 on them. I wouldn't believe any "rumor" that the the 3GS will be *immediately* unsupported when iOS 5 arrives.