According to Famitsu, Pokemon Platinum sold 967,675 copies in its first two days in stores (September 13th through 14th), shattering all previous Nintendo DS records to become the fastest selling DS game in history. On its third day, it surpassed the one million mark by leaps and bounds.
For comparison, Diamond and Pearl combined sold 1.58 million units during their first week. At the rate Platinum is selling, it may break that record too. To this day, Diamond and Pearl has sold an impressive 5,571,456 copies, and Platinum is already about a fifth there in only its first two days. (Though of course, sales slow down each passing day, so it may even-out in the end. Still, the game is selling much stronger than D/P did during its first few days, so it will probably surpass them and to a quicker degree.)
But remember everyone! Pokemon is dead, and you should be ashamed for liking it! Pfft, at the rate this game is selling, your friends will probably secretly own it in the future too. To read our extensive coverage of Platinum, you can check out these two, very long news stories: Pokemon Platinum Coverage [Sept. 10-12], Continuing Pokemon Platinum Coverage [Sept. 12-16].