What's missing from iPhone 3G and iOS4 update.

What's there
•Folders
•Threaded mail
•iTunes playlist creation / editing
•5x digital zoom
•iBooks -- PDFs work, too.
•Photos: you can now sort by albums and places
•Spell check has been updated
•Spotlight search: you can search the web and Wikipedia from the home screen search bar.

What isn't
•Multitasking: this is the big one. Double-tapping the home button doesn't give you the handy menu, there's no fast app switching, and background use is missing in action.
•Screen lock: normally in the multitasking menu (which isn't available on 3G), it's nowhere to be found, even under Settings. Bummer.
•Home screen wallpaper: you'd think this simple addition would make the cut, but sadly no. The dock has been given a visual upgrade, though.
•Bluetooth Keyboard support: we haven't tried this ourselves, but an influx of reports from Twitter suggest this is also a no-go.
http://www.engadget.com/2010/06/21/iphone-3g-ios-4-and-you-whats-missing-spoiler-multitaski/