DS and PSP neck and neck in sales?

According to Next-Generation, the Nintendo DS and PSP are neck and neck in sales. From the article: "Nintendo has continued its 'good news' publicity flurry with European and worldwide sales stats for Nintendo DS. But it looks like PSP is gaining ground... In December, SCEA's Kaz Hirai said PSP's U.S installed base would be doubled over the peak Holiday season, to six million. If achieved, (and assuming all released figures are accurate) PSP's global figures may well be close to matching DS's."

Only until Sony officially releases sales data will we know for sure. But if true, it would be interesting to discover how much of the PSP's sales are credited to other non-game functionality.