Are you upgrading to OS 4? I already did using the Beta 1 and it's pretty usable and no lag (iPhone 3Gs & iPod 64GB)
Unless your name the Spaz, you'll probably upgrade.
If I'm still on the iPhone, I'll upgrade (I'm playing with the Nexus One)
Once there's a jailbreak, absolutely.
Mind me correcting myself and others?
OS 4 has been jailbroken already. Dev Team keeps under wraps (just like Apple secrecy) the new bugs and exploits they find. Pay off is that now, we have a working OS 4 jailbreak.