Can I use my iphone while in Japan, Hawaii, and Alaska?

I'm going on a trip and I want to know how much I can use my phone without getting charged extra. Now I have a US iPhone 4 on at&t, since Hawaii and Alaska are states I won't have problems using it there right? While in Japan do I have to completely avoid calling, texting, and using my data plan because of extra charges?

You need to call AT&T customer service and ask because it depends on your cellular plan.

If you are on a national calling plan, Alaska and Hawaii should be fine.
Although your iPhone might work in Japan, it will be roaming. Just turning it on will cause it to start downloading and it will cost you a bundle. You want to call ATT and talk with them about your options. One option is to turn off the data part of your phone while you are there. I know a guy who turned on his phone and within minutes had added $300 to his bill because his phone automatically downloaded his email.
Don't ask about this at the store, call them. The national folks know a lot more about it and can give you good advice. They do mess up, though, on which phones work in Japan. They have different technology and the "world phones" don't always work there.
If you will be in Japan for awhile, there are companies which will rent you a phone. I rented one off of the JTB (Japan Travel Bureau) site and they sent it to my home before departing the USA. You can also rent one in Narita airport, but it's more expensive. They don't have prepay phones in Japan unless you sign a contract, so it makes no sense for someone on a short trip.
Have fun!

If you take your iPhone to Japan it will work but it will also cost an arm and a leg to use it; the charges are per min and obscene. If you call At&t to add an international pakage you will get different answers from every rep you talk to about the coverage (it's not their bill so they don't really care), so I suggest reading the web site instead and only calling if you need clarification on anything (you might need to talk to a supervisor, rep's don't always know what they are talking about). I would say take your laptop and use skype for free if you can and buy a phone card for the pay phones in Japan for local calls. I used my iPhone very little in Japan and my bill was over $500.00 dollars after only 2 weeks but if you yell at them enough they WILL credit it ALL! I got all the carges removed!