Tight supply of (other) key Wii parts is blamed for the scheduling push-back.
Because of the high demand and not-as-high supply of components such as integrated circuit chips and PCBs (printed circuit boards), Nintendo has had to delay a ramp-up of Wii console production, originally scheduled for June of this year.
In Nintendo's Q2 2007 earnings report, the company had stated that it would be increasing total Wii shipments from 14 million up to 16.5 million units in their current fiscal year, ending March 2008. It's not exactly clear if this scheduled production increase was meant to account for the additional 2.5 million Wii consoles. However, the financial report stating this increase was released at the end of July, a month after the scheduled increase was set to begin. Perhaps this means Nintendo was planning an additional increase on top of the one previously announced?