Carmack IDs Quake Live on GameSpot is a Q&A with the id Software Technical Director discussing their upcoming Quake Live project. Along the way he clarifies where this does and does not depart from Quake III Arena:

The core game has had very little changes to the engine. The idea was to wrap this really state-of-the-art--player interaction, score-boarding, leaderboards, chat forums--all this stuff, around a stable, consistent game. So there are some advantages to that, in that every computer that anybody is at all going to sit down on, and is connected to the Internet, can play Quake Arena well. You know, most of them run it at 120-something frames per second on there. There are no driver issues, because everybody used this game as a benchmark for five years before it was superseded by other things on there.

The data has actually been touched on every single level, though. While the core rendering engine has only modest tweaks on it, nothing particularly to improve what you really call rendering things. But every map has been touched where there's a level of polish that modern games have that, when we look back nine years ago, standards were a lot more lax. I mean, Quake Arena was the first mainstream, hardware-accelerated game, and it was much more about, "Wow, look at this. Isn't it cool?" And people weren't looking at all the details, like making sure the textures line up or the lighting is appropriate.

And we have had the designers make a path through every single level in the original Quake Arena, the Team Arena content, and there's a little bit of custom new stuff that goes on in here also. So everything is cleaned up, brought up to a modern level of polish, the billboards and advertising scoreboards are very nicely integrated. It's not a matter of just finding a bare spot of wall and throwing up two triangles that you can stream an ad on to. They're actually built into the levels in a very stylish way, where you've got the boards with spotlights, lighting them up as things change. And all that's been done really nicely.

But, you know, 90 percent of the work on the project has been stuff that goes around the game--the Web site experience, the game rankings, all the stat-tracking and database management, and all of this stuff that really is better than probably any game, anywhere has done. We went out, we surveyed the landscape of what was available, what we could integrate, what we could take, and what we could improve.

And we make no claims or try to hide the fact that the core of the game is essentially nine year old technology. But everything around it is modern, 2008, state-of-the-art stuff.