Don’t install today’s iTunes 9.2 if you plan on using Spirit for fresh JB (but it’s fine for PwnageTool & redsn0w users)
Apple will also be releasing iOS4 for iPhone 3GS, iPhone 3G and the iPod Touch shortly. iOS4 will be made available as a free update and can only be installed using iTunes 9.2, so if you have used Spirit to jailbreak your device you will have to wait till an update is available to it.
However, the iOS4 has already been jailbroken using PwnageTool Bundle. In addition to that, there is also a iOS4 unlock through Blacksn0w RC2.