I installed 4.0 a few days ago on my 3G and only notable difference is the folders. I think to use multi-tasking I need to upgrade my phone to 3gs? Stinks if that's correct!
Of course, no apps background properly currently, but it's useful for testing.
I'm using it on my primary iPhone, but I carry a backup phone in case I actually need to get stuff done ;)
I only have one iPhone, but I do have iTouch I use to test our app on.
http://macreviewzone.com/archive/hardcider/Reviews/RD9.shtml