iPhone and iPod Touch Compatibility With iOS 4.0

• iPhone 4: Full iOS 4.0 features
• iPhone 3GS: Full iOS 4.0 compatible except iPhone 4 specific features
• iPhone 3G: Partial iOS 4.0 support. Doesn’t support multitasking, wallpaper and Bluetooth keyboard.
• iPhone 2G: No iOS 4.0 support. (Custom Modded 3.1.3 with iOS 4 features available)
• iPod Touch 3G: Full iOS 4.0 support except for iPhone specific features
• iPod Touch 2G: Partial iOS 4.0 support. Doesn’t support multitasking, wallpaper and Bluetooth keyboard.
• iPod Touch 1G: No iOS 4.0 support.

Source: iOS 4 Update For iPhone 3GS, iPhone 3G, iPhone 2G and iPod touch 3G, 2G and 1G