Link to press release
The Nintendo 3DS has reached sales of over 5 million in Japan faster than any system in history. If you look at the above graph you will see that in just 51 weeks the 3DS has outclassed even the mighty DS and Gameboy Advance.
The aggressive price change combined with the great games really killed it for Nintendo. The sales are only going to get better from here forward with more heavy hitting games that are coming this year and the next.