Cube.ign.com is running a story saying that Nintendo is claiming about $100 million worth of GameCube sales on their launch day:

Though Nintendo shipped 700,000 hardware units, the firm would not offer specifics regarding exactly how many of the initial batches have been sold. It's over what we think Microsoft's allocation was, NOA vice president of corporate affairs Perrin Kaplan said.

In addition, Shacknews is running a Business Wire story that seems to be the source of the twice Xbox sales quote:

Movie industry sources put ticket receipts for the three-day opening weekend of Harry Potter and the Sorcerer's Stone at $93.5 million. But combined U.S. retail revenues for NINTENDO GAMECUBE (including hardware systems, games and accessories) reached more than $98 million in just its first single day of availability on Sunday, November 18.

The NINTENDO GAMECUBE also represented the top video game launch of 2001, with initial hardware unit sales running more than twice that of the Microsoft Xbox, and even Nintendo's own wildly successful Game Boy® Advance, which debuted in June. Many stores reported selling out of the hardware system in a matter of hours.

The emphasis is ours. The article also indicates that the initial GameCube shipment of 700,000 may be sold out by the end of this weekend.