I had a quick read through the DisplayPort spec (at vesa.org). Apparently the spec includes some bits about wiring up the connectors a certain way so that the computer can recognize when an adapter is plugged in and what type. It can then signal the video circuitry to output a different type of signal (HDMI or DVI or VGA or whatever) out of the same pins. So, these DisplayPort->DVI or DisplayPort->VGA adapters should be fairly standard beasts, and work on any DisplayPort equipped computer, assuming the underlying graphics hardware supports HDMI or VGA or whatever it is you're trying to get out of the port.

This is all completely disregarding the physical incompatibility introduced by the "mini" DisplayPort connector (a connector which is completely absent from the spec, by the way). I don't fault Apple for wanting a smaller connector. Their connector seems much better for laptops than the bigger standard connector. That said, any hopes for interoperability depend on it either getting into the next rev of the spec, or Apple selling so much equipment that the market forces its adoption.