So it turns out that Rage didn't ship with mod tools after all. It's only taken id Software... a year, four months, and four days to deliver, but at long last, according to the official Bethesda Blog, the Rage Tool Kit will be arriving on Steam later today. You'll be able to grab it in the "Tools" section of your Steam client.
The Rage Tool Kit will be a "35GB+" download, and it will include the idStudio "world building tool" as well as tutorials, Rage maps, and game assets. According to the "welcome" PDF linked by Bethesda, the kit includes id Software's own development tools—and those are not for the faint hearted:
Keep in mind that this is our internal tech used to create RAGE and should not be interpreted as consumer-ready. This download is provided on an “as is” basis only for the technically sophisticated and adventurous!
In a similar vein, id Software's John Carmack tweeted this morning, "Doing significant work will require patience, because internally we use a 300 core renderfarm for megatexture creation." Yikes. The welcome PDF corroborates this, quoting typical build times of 20-60 minutes for initial map loads and 1-3 minutes for subsequent loads. I believe you get a full 3D preview of levels in the editor itself, though. At least, that was the case when Carmack demoed the tools a few years ago:
Late or not, I'm glad id Software is finally about to deliver. Modding is one of the main things that makes the PC a great gaming platform. Mod tools also prolong the longevity of games, and they give aspiring designers and developers a way to hone their skills.