Yep. I spend all night getting the Pwnage jailbreak to finally work with my 3GS and iOS 4.0. That all works and MyFi doesn't work nor does PDAnet. So my iPad won't tether. I'm hoping that this doesn't take long to get updated. I haven't really found a good reason to upgrade to iOS 4.0 if you have a 3GS, 3G or 2G.
Multitasking - only for apps designed to work with multi-tasking. Not many, that's for sure.
Folders - nice, but not worth the aggravation.
Custom springboard - if you've jailbroken, we've had that for quite some time.
Better Mail - don't see it personally.
iBooks - nice on the iPad - iPhone is too small.
All-in-all, you're currently better off with a jailbroken 3.1.2 or 3.1.3. Once you do upgrade, you can go down, but I was never able to get my carrier support back. Unless you're crazy, like me, wait a while longer before upgrading.