Ever since the announcement of the iPhone 3G, many people around the world asked the question: "Will it be unlockable like the regular iPhone?" The answer was no, at least not with the firmware that is was shipped with. Apple made sure to make it more difficult to unlock the iPhone and it's many features so that it can control what the users could and could not do, also making it available through certain carriers.
Many phones are designed to work with a single service carrier, such is the case of the iPhone 3G. The methods that were developed to unlock the iPhone 3G were able to remove the carrier restriction, along with unlocking many new features. The process was not as easily exploited as with the original iPhone.
Only 4 days after the initial release of the iPhone 3G, it was unlocked by the use of SIM cards that tricked the phone into thinking it had a special SIM card used to test various functions of the iPhone 3G's capabilities. This has evolved into hardware solutions that are cheap and readily available through many sources.
It wasn't until later on that someone figured out how to use software methods to unlock the iPhone 3G. Fully unlocking the 3G took time and patience to crack. Some companies are still trying to figure out how to unlock the 3G through a software type solution to profit from the phone's success and locked features.
Because of Apple's monopolizing practices, there are not very many ways to unlock an iPhone 3G without spending money unless you are willing to take the time to find someone that has created some way to unlock it free of charge. Those people that have figured out how the iPhone 3G works and how to unlock them are appreciated by all of the consumers who got the 3G for all of it's capabilities.
Eventually there might be a free solution to all of the iPhone 3G unlocking problems. For now, there are cheap services that can help to unlock your iPhone 3G with little or no consequence on your part. The future holds many surprises, maybe they will release a better iPhone or Apple product soon enough that unlocking the iPhone 3G will be something everyone could do inexpensively.