Here is a very, very good article on DisplayPort.
Let me highlight a few things that will help shed light on Apple's new 24" LED Cinema Display with DisplayPort:
1. DisplayPort uses a packet data architecture (like Firewire and Ethernet) which is very different from HDMI or DVI.
2. DisplayPort carries audio, video, and data, all bi-directionally.
3. DisplayPort can send a DVI-type signal for conventional displays.
4. DisplayPort is designed to handle internal as well as external connectors. It's practically custom-made for laptops, with the ability to carry commands to devices such as backlight (for brightness), status lights, etc over its data channel, without adding more wires.
It's now clear to me that DisplayPort is not just another variant of DVI. (HDMI is basically DVI + audio + a specific encryption protocol, and it's not bidirectional.) This means that it could be very challenging to make the new 24" LED Cinema Display work with older MacBooks and MacBook Pros.
Not impossible, surely -- remember the $400 DVI-to-ADC adapters sold before Apple started selling their own? -- but not a simple physical adapter like Firewire 800-to-400. And, probably not cheap either, unless someone like Apple themselves throw enough business to a manufacturer to amortize the development cost across a lot more units than most niche players can expect to sell.
(Thanks to Daring Fireball for the link.)