Can Apple unlock my iPhone 3GS?

Not without a request from whatever cell provider you are with.

You would need to check with your cell provide, but mine did it for free.

I just had to fill in an online form and about 3 or 4 days later, it was unlocked.

jerry128 wrote:

If I ask att, they would do it for free?

No, AT&T does not unlock iPhones for any reason for any one.

This question has been asked and answered a few billion times. A simple search would have given you the answer.

why not just buy an unlocked iPhone 4??

From the horses mouth... http://www.att.com/esupport/article.jsp?sid=KB82027&cv=820#fbid=DJ6UfmHm2cL