Keen to get iOS4 on your iPhone before Apple officially release it on June 21st? Turns out, with the iOS4 Golden Master (GM) release in the wild - which Apple pushed out to developers after Monday's WWDC keynote - those of us with iPhone 3G or 3GS handsets can now take advantage of the new functionality, and thanks to a little hacking you can also jailbreak an iOS4-running iPhone 3G and enable the custom wallpapers and multitasking Apple don't want you to have.
Pixelated Geek run through all the instructions, and it looks like a relatively straightforward process getting iOS4 onto your device. After that, there are a few - slightly more complex - tweaks you need to do in order to get custom wallpapers and multitasking support on the 3G; of course, the 3GS supports those natively.
Unfortunately there's currently no jailbreak for the 3GS running iOS4, so if you're reliant on an app or two through the Cydia unofficial app store then you may want to hold out until the iPhone Dev Team push out a new release. That's been promised in time for the iPhone 4's debut on June 24th.