The Idolm@ster series of games is part of a Japan-only major multimedia project, including CD, games, DVD, books, radio, and concerts, called Project iM@S. Bandai-Namco Games sadly has no plans to release Idolm@ster in North America, which is what they tell us whenever they are asked about it (since 2005). Thus, to play Idolm@ster, you must import from overseas.
The tricky part of importing is that in the video game world, sometimes there is a region-lockout and sometimes there isn't. This article discusses what kind of hardware you will need to play each game in the Idolm@ster series.
Xbox 360 (The Idolm@ster, Live For You!, The Idolm@ster 2) Edit
The situation on the 360 is straightforward: all of the Xbox 360 Idolm@ster games are locked to the NTSC-J region, no matter whether you buy the "Japan version" or the "Asia version".
The Xbox 360's region lockout is robust and there's no known way to bypass it, even if you took to the legal gray area of modifying your console. So you will need to purchase a Japanese-region 360 console to play. You can buy a console in Akihabara during a trip to Japan, or import one from PlayAsia, NCSX, or other importing companies. Shop around. It's a non-trivial investment but access to the Idolm@ster games (and Cave shooters) just might be worth it.
In case you're curious, if you try to play iM@S on a US-region console here's what happens. The 360 will read the game and show it on the dashboard as you'd expect, but will display an error message if you try to actually boot it up for play. This won't harm your console or your save data; you just won't be able to boot up iM@S.
The Japanese console can be set to whatever dashboard language you want, including English, so you won't have trouble navigating. You can also use your US gamertag on your Japanese console if you want to keep your achievements all together. Swapping a hard drive back and forth or recovering your gamertag each time is possible. Region locking only applies to the game bootup; all other content is compatible without issue. The easiest way to handle multiple consoles though is to put your gamertag on a USB thumbdrive (now that the Xbox360 supports them) and move that back and forth as necessary.
Additional gamertag-sharing and DLC licensing strategies are discussed on the Downloadable Content page.
PlayStation 3 (The Idolm@ster 2, Gravure For You) Edit
Now that the PS3 version of iM@S2 has come out and we have iM@S on the PS3 for the first time, we know the answer to this question. iM@S2 is region-free and will work on US PS3s!
Until the game came out, there actually was a question about this status. PS3 games didn't have region coding for many years of the console's life, so the default assumption was that it should work. But recently Sony patched a region coding option into the PS3 firmware. It seems iM@S is not taking advantage of that feature at present.
PlayStation Portable (The Idolm@ster SP) Edit
Good news; no region lock! All versions of iM@S SP sold on the UMD discs will work with US PSP handhelds with no extra work required. There are three different versions - Wandering Star, Missing Moon, and Perfect Sun - each with a different set of three idols and a different rival. They do share save data and DLC though.
The video and audio quality might be lower than what you see in Xbox360 videos (for instance stage performance videos are single idols only), but this is definitely the cheapest way to try out iM@S to see if it is the kind of game for you.
Nintendo DS (Idolm@ster Dearly Stars) Edit
Dearly Stars is a DS-i enhanced game, so what you need to do here depends on what kind of DS handheld you own. The DS-Lite and earlier have no region protection, while the DS-i and later do have a lockout. You can play the game on a US DS-Lite and just skip out on the (not really important) DS-i camera features. Or if you want to use the DS-i features you will need a Japanese-region DS-i or later model.
Electricity Format Edit
Should you end up buying imported hardware, you'll need to be sure you can run it in your country. Japan uses both 50Hz / 100V and 60Hz / 120V electric formats, so the country's consumer electronics are usually designed to work with a range of electricity types. A Japanese X360 or PS3 console will work just fine in the United States, which uses the 60Hz / 120V format and the same basic wall plug.
For other countries, you may need a transformer to convert the voltage and/or a special plug to convert the prong layout.