The issue isn’t with the 3GS it’s with the 3G. My daughter and I had similar phones (she has since gotten a new one because hers was so frustrating after the ‘upgrade’ to 4.0) and I can tell you this is not BS.
My iphone has not run well since the upgrade and 4.1 was only a minor improvement. It has been a major irritation and what I considered a great phone has been rendered agonizing slow, requiring multiple daily reboots to get apps to run.
I would not upgrade if I had to do it over.
I can attest to the sluggishness of my 3G after the upgrade. It was certainly not a brick though. I upgraded to the 4, but not because of the sluggishness but rather I wanted access to FaceTime. I certainly don’t think it is anything worthy of a class-action lawsuit, in a sane world that is.