iPhone OS 4.0 jailbreak has a few major caveats

If you are iphone user but you don't feel comfortable by being locked to one network or limited to using only approved software then jailbreaking the device is created for you.

We have to admit that with each update the jailbreak crowd has to find another way around the lockdowns

Apple puts into effect, and Jailbreak artist MuscleNerd is already at work as the iPhone OS 4.0 is coming soon.
A wireless iPhone Jailbreak allows an iPhone user to unlock one's phone without connecting to a computer. This makes unlocking an iPhone easier than ever before.
Jailbreaking an Apple iPhone allows access to all compatible mobile providers so you are free to access other services. Jailbreaking is often done to maximize the performance from a handset although often at a sacrifice in performance, especially battery life. However, any upgrade to the operating system will likely remove any jailbreak and a newer jailbreak will be required. Also it is available to buy a jailbroken phone or do it yourself.

But in spite of above mentioned here are some caveats to take into the consideration:

First, the jailbreak only works on the iPhone 3G.

The second is that the jailbreak software is only for developers.

The third caveat is that reports are the jailbreak is unstable now with reports of wireless connectivity issues.

Still, this is certainly a start and we can expect things to get better.

Whether jailbreaking is legal or ethical depends upon your terms of service although Apple no doubt does not sanction such actions.