I was also recently told this by Apple Technical Support when I called to find out why the USB socket on the mini DisplayPort Dual Link adapter fails to maintain a reliable connection to attached USB devices. The Technician "was told by Apple engineers that the Dual Link adapter only works with Apple displays."
This isn't a solution to the problem. It's an excuse.
What I find most aggravating about this is that no one else manufactures a mini DisplayPort Dual Link adapter!!! The previous version of the MacBook Pro did not require an adapter to connect to a dual link display.
This is so reminiscent of the infamous Apple Display Connector that I want to puke. I upgraded from a heavily-modified, 8 year old G4 Cube (with ADC) to this unibody laptop, thinking everything would work out fine. But as it is, I'm right back where I started: an obscure, non-universal connector locking Apple computer owners into Apple peripherals.