MuscleNerd released a warning last night warning iPhone 3GS/iPod Touch 3G owners not to update to iOS 4.0, especially those who used Spirit to jailbreak their phone for the first time. Unfortunately, MuscleNerd did not provide any information regarding this warning, but if you have an iPhone 3GS or an iPod Touch 3G, you should not venture to iOS 4.0 yet.
For the rest of the devices, there is no warning, so you can probably switch to iOS 4.0 without any problem.
