The 3DS is the first Nintendo handheld to pose region restrictions on all of its games from other regions. This keeps those who want to play games from Japan, for example, from doing so unless you own a 3DS device from Japan.
A group of 3DS owners think this is wrong and have found a way to get around the handheld's region-locking. The method is not a hack that will let you do anything (like play roms downloaded off the Internet, for example) but play games from other regions.
You can learn about their method for making this happened on GBATemp. The method is a little more involved than what most users are used to, but if you are willing to put in the extra effort, you'll reap the rewards.
Nintendo has not publicly commented on this story.