If you do have to hold onto your 3G for awhile, you have to deal with a secondary issue also confronted by original iPhone users last year with iPhone OS 3 -- is it worth upgrading to iOS 4?
Steve Jobs was upfront about the fact that not all of the new features will work on the 3G, but until yesterday we weren't quite sure exactly which features beyond multitasking would work or not. Engadget has a comprehensive list of what works and what doesn't.
What you get: Utilize folders, threaded mail, playlist creation/editing in iTunes, digital zoom (just don't expect good results), iBooks including PDF support, Photos, the Game Center (once activated), updated spell check, and Spotlight search.
What doesn't work: Multitasking, fast app switching, screen lock, wallpapers and Bluetooth keyboard support. You know, the good stuff.
If you're planning to upgrade your phone shortly, it's probably not worth the hassle of upgrading your 3G. If you do plan to hang onto your phone for a little while, it might be worth it just to gain some of the features listed above. I plan to upgrade my phone to take advantage of the mail changes, iBooks and folders. To those out there with your 3G iPhones, what do you plan to do with your phone?