Like I said, read the article on EETimes about the launch of the 48xx's and the GTX200's: here

Some quotes:
"We didn't want to come out with one monolithic GPU and then disable parts of it for different markets," said an AMD spokesman prior to a full disclosure of the part in a briefing in San Francisco on June 16.

The strategy makes sense for the financially troubled AMD which also has laid out conservative road maps for its computer processors. The graphics choice reduces costs and risks while maximizing returns for the company which has suffered through multiple loss-making quarters.

The decision to use a two-chip strategy for the high end was made more than two years ago, based on an analysis of yields and scalability. It was not related to AMD's recent financial woes, said Rick Bergman, general manager of AMD's graphics division.

"I predict our competitor will go down the same path for its next GPU once they see this," Bergman said. "They have made their last monolithic GPU."
I have definitely heard rumblings that Nvidia is moving towards more of an ATI-esque approach to this, though to what extent I do not know. I know that most GPU manufacturers lay the specs for generations long in advance (for example, G80 was supposedly designed years before they ever went into actual production) so that part I do believe in, but I also believe their economic situation + failure of the R600 pushed them down this path a lot faster/deeper than anticipated.