UPDATE 28 JUL
if you want to unlock your iphone 3g with the new firmware 3.0, first jailbreak using this tutorial to unlock iphone 3.0
then, add the repo: repo666.ultrasn0w.com to Cydia or Icy
install ultrasn0w and reboot your iphone.
thats it.

UPDATE 04 Dic
Yes!! Now the guys at iphone-dev have found a way to unlock the iphone 3g without opening it. Here´s the guide, hope you understand it. if not, tell me in the comments, i will try to be as clear as i can.
You will need your iphone 3g jailbroken with the latest firmware at the time (2.2) if you already have this, restore it via itunes, and then go to the step #3.
Step #1: You will need to update your iphone to the latest firmware. You can do this with itunes, connect your iphone 3g, and choose update. This will update also your baseband, that´s what we need.
step #2: After you have updated your iphone 3g, you will need to jailbreak it with quickpwn. here you can find the guide to use quickpwn, use the same steps as the iphone.
step #3: When your iphone is jailbroken, its time to unlock it. You can use installer or cydia, add the following source:
CYDIA: http://apt9.yellowsn0w.com
installer: http://i.yellowsn0w.com
install it, and reboot your iphone with the sim installed. Wait about 90 seconds, and if you can´t still see your carrier, pull the Sim and try again.

There you have, your iphone 3g unlocked. Im waiting for your questions on the comments. if you have sucessfully unlocked it, let me know in the comments, thats the way to pay me for writing this guide. i aprecciate it a lot. it motivates me writing new guides.

UPDATE 16 NOV
Note to some stupid commenters: the post that is below the line, was written before the iphone 3g came out. Its just the reality. Now here are the latest news about the iphone 3g unlock.
Yes, its posible to unlock your iphone 3g, with a simple sim adapter, you can unlock it. it makes that the iphone thinks that it operates on a fake carrier.
No, there isnt a software solution to unlock the iphone 3g. DevTeam has posted that they are working on it, but they don´t promise anything, there is no date to release this hack. Here is The software solution to unlock the iphone 2g.
That´s all you need to read guys, the post below the line are just past thoughs.
you can get the sim to unlock your iphone 3g here.
see you later!!!
keep subscribed to the feed if you want to know when the iphone 3g unlock solution is going to be posted!

More Links

Now you have your iphone 3g. Now, you want to use it, you want to unlock it. First, lets get some points:
The 3g is a paid service. You must have a plan with your carrier if you want to use 3g technology.

The 3g coverage is very poor. You must live on a crowded city or something sometimes if you want 3g technology available. Otherwise, you will not be able to use 3g internet on your iphone.
The iphone 3g still doesnt support video conference. Its the same as the iphone, but with more battery power, gps technology, thinner, and 3g. And it also support all kinds of 3.5mm earphones jacks.
Now, think about it. AT&T this time is angry with the customers. You wont be able to activate your iphone via itunes or something, you will have to go to the at&t store and they will take 15 min of your time (+ the time you wait in line). Now you are thinking, that the hackers will be able to unlock it in someway, well, if you dont activate it after 30 days of your purchase, at&t will charge some money in your bill. Thats it, at&t wants know all the revenue! they are loosing money every time one of us purchases an iphone, but they recover it because you have to sign up a contract for at least 2 years.
Now, what if you want to change your iphone 1.0 to the 3g? if you are in a carrier plan right know, you will have to renew your contract, your contract will be reset. You will have to get your contract with your phone carrier for another 2 years.
Yes, the iphone is going worldwide, but it wont be available for activating by yourself you know?

apple now is being smarter, the iphone is hard coded, and now the things are different. You can get a penalty if apple knows that you have illegal software in your iphone. there is an apple kill switch to turn off apps remotely on your iphone.

So, i dont know, i will better stick with my first generation iphone. Anyway, in my city, there isnt 3g technology, nor gps.

What are your views? What do you think about this new way to sell iphones from apple?
They want a monopoly, maybe. for 199$ i would better buy an iphone than another cellphone. But.. the 3g plans are very expensive, starting in some countries from 60$ a month!!
yes, your iphone at the end of 2 years will have make you spent about 1200$ bucks man!!