Do you think anyone really knows the answer to that?
I am not having any lagging or hang-ups myself! 3GS is working great! I have been using it all day listening to music in the background, checking email, texting, taking some notes in a meeting, replying to some email. I am loving it!
4.0 was just released yesterday.
It'll most likely be another 2-3 months before 4.1 is released.
Now, there will be small 4.0.1...4.0.3...etc releases to fix any issues.
Just hang tight. I'm sure Apple is already working on the next OS update.