Can I use my iPhone 3G in Japan?

I'm moving to Japan January 2010 to stay there minimum 5 months and I of course need a mobile for staying connected to the world.
The most important feature is Skypem which means I need something like 500MB data per month. I willing to change phone (an Android fx.) but I'm not sure wether or not I should buy it in Japan or at here in Denmark.
Anyways, my questions go like this:
1. Is it possible to subscribe to a data plan for only 5 months? Without being ruined?
2. Is it possible to buy an high-end phone in Japan without a 2 year contract and then subscribe to a data plan?
3. Kinda technical question; an iPhone 3G supports (surprise 3G), but it consumes a lot of power. Therefor I normally have 3G switched off to consume power and I instead rely on EDGE. It's a bit slow but it works. How is it with data networks in Japan? Is it only 3G? Can I use EDGE? Is there some kind of proprietary network that only phones bought in Japan support?

I'm not quite sure if your iPhone will work here or not. I don't know how that goes, but I do know that I just purchased an iPhone over here and am able to use it just like you can back in the states. They DO have the 3G network here. I don't know about EDGE either, but if it's an app, you should be able to use it just like you would any app. The two major carriers here on Okinawa are AU and Softbank. You may be able to contact one of the carriers, especially the ones around American military bases and American PXs...They have employees that speak pretty good English who should be able to help you. It seems like it should be able to be used with a different carrier if you change the SIM card? I'm sorry, I know my answer wasn't much help, but hope someone else comes along that knows more than I do.

so yes, it will work in Japan. EDGE however is a GSM technology and will not work (the Japanese skipped GSM and moved directly to 3G). You can subscribe for only 6 months I think, but it is often cheaper to subscribe for two years and then pay the penalty, if you are getting a new phone. The store personnel will happily calculate the cheapest method for you. If you will keep using your iPhone, make sure it is unlocked! Also, many stores (generally those operated by contractors, and not directly by softbank, docomo etc), will not sell you a contract without a phone. Consider getting the iPhone 3GS, its faster than the HTC Magic and way faster than the old 3G. YOu will then also get Japan only specialities like the emoticons, without jailbreaking. Of course it can still be unlocked by the usual method.

(1) Not in the sense you are looking for.
(2) nope
(3) will only work with 3g.

Sorry to be so frank

As I understand it there are multiple, but no easy solution for buying a mobile contract in my situation.
If you were me, would you then bring your current 3G phone to Japan and see if you could find a reasonable priced contract? or would you sell it before going to Japan and then buy a new phone including contract in Japan (and risk paying a penalty if you leave the country before the contracts ends)?

1. Is it possible to subscribe to a data plan for only 5 months? Without being ruined?

As Kevin points out, you are probably looking for a short term subscription plan to cover the 5 months you are here, but these are not available for less than a 2 year contract. You could consider going with a rental sim but the costs are probably prohibitive (105 yen per day for the rental, 105 yen/min local calls, 1 yen/128 bytes mms, .32 yen/128 bytes internet)

http://www.softbank-rental.jp/en/index.php

2. Is it possible to buy an high-end phone in Japan without a 2 year contract and then subscribe to a data plan?

The carrier websites are unclear about this but it may be possible. All the major cell phone carriers allow you to purchase the handset outright and then get service without a contract. What wasn't clear was whether or not they allow you to get an unlimited data plan without a contract. This may be worth looking into.

3. Kinda technical question; an iPhone 3G supports (surprise 3G), but it consumes a lot of power. Therefor I normally have 3G switched off to consume power and I instead rely on EDGE. It's a bit slow but it works. How is it with data networks in Japan? Is it only 3G?

Yes, it is only 3g in Japan

Can I use EDGE? Is there some kind of proprietary network that only phones bought in Japan support?

As metioned, there is no Edge network in Japan. Turning off 3g takes your phone off the network entirely.

For a short stay of 5 months it may be cost prohibitive to get service that would give you the full functionality of your iphone. And switching to an android phone doesn't offer any advantages.

Instead I would suggest getting a cheap prepaid phone to handle your emailing and local calls while here. Handsets start at 5000 yen, and credit can be purchased in 3000 or 5000 yen increments. You could then use your iphone basically as an ipod touch to access the internet at a (rarely free/public) wireless hotspot to make your skype calls. Skype mobile doesn't work over 3g anyway so it doesn't seem like you will lose too much functionality.

If you were me, would you then bring your current 3G phone to Japan and see if you could find a reasonable priced contract?

As mentioned, for 5 months there are no contracts available. And to update my previous answer, I just found on softbank's site that iphone service for example requires a 2 year contract. To get a data plan on another carrier without a contract is highly unlikely, especially as they don't support the iphone.

or would you sell it before going to Japan and then buy a new phone including contract in Japan (and risk paying a penalty if you leave the country before the contracts ends)?

If you got a contract only phone its not a question of risking a cancellation fee, you would definitely pay a cancellation fee as well as the cost of the phone at full price.

However, if you got an 8gb iphone 3g it would cost you about 35000 yen to leave your contract after 5 months (balance of the cost of the handset plus cancellation fee). Used iphones sell for around 30000 yen on ebay so this may be a viable option.