redsn0w 0.9.5 beta will turn on multitasking and homescreen wallpaper by default. You can also deselect these features if you don’t want them.
Please make sure to read the Dev teams note before using redsn0w 0.9.5 beta:
YOU SHOULD STAY CLEAR OF THIS BETA SOFTWARE IF YOU RELY ON A CARRIER UNLOCK. That’s because the beta redsn0w works only if your device has already been upgraded to the stock 4.0beta1/beta4/GM IPSW from Apple, which contains a baseband update. If you installed that IPSW, you’ve already lost the carrier unlock until the next planned release of ultrasn0w and blacksn0w.
You can also jailbreak iPhone 3GS running iOS4 using PwnageTool. You will require to download iTunes 9.2 in order to install iOS4. However, if you have jailbroken your iPhone or iPod touch using the Spirit jailbreak tool, do not upgrade to iTunes 9.2 as of yet.
More instructions and the download for redsn0w 0.9.5 Beta for Windows can be found here.