I'm not upgrading YET but I have a question

There is a list of what is not supported.

Multitasking with 3rd party apps is not supported, nor is the ability to set a photo as a home screen wallpaper or background.

The folder feature for apps is supported with iOS4 for the 3G.

This is what is included.

http://www.apple.com/iphone/softwareupdate/

iOS 4 works with iPhone 4, iPhone 3GS, and iPhone 3G. Not all features are compatible with all devices. For example, multitasking is available only with iPhone 4 and iPhone 3GS.

Multitasking with 3rd party apps is not supported with iOS4 on the 3G nor is nor is the ability to set a photo as a home screen wallpaper or background.

It's not worth it on the 3G. Without multitasking the other changes are in the noise. The slow response, crashing and other issues with OS4 on the 3G phone for some (like me)are significant. I painfully reverted back to to OS3 and am much happier now. The phone is so much faster, apps open quickly--hey it's as though I have multitasking.

OS4 may be a better experience on the 3GS that allows the more advanced functionality but I have heard only dismal things with OS4 running on the 3G.

Be very careful--the 3G phone work great without OS4.

Cheers

Reportedly this is being addressed for the 3G with 4.1 which has been released to developers as a BETA for testing and feedback.