Sean Barret's GDC 2008 Sparse Virtual Texture presentation was his attempt to recreate what Carmack was doing at the time. There's source, too. There has been some correspondence between Carmack and Barrett since then.
(The presentation is a bit dense, but worth taking the time to understand -- I spent a few hour-long sessions with a whiteboard after watching it twice in the course of two days, and eventually got to the point where I think I understood it all. Much easier if you have a fragment shader background, which I didn't at the time. It's also really neat that the presentation is the demo.)
+1 to zacharmarz for mentioning the excellent GDC2011 Mega Meshes presentation by the Lionhead folks working on Milo and Kate. There are a lot of very practical solutions mentioned therein, for example: rendering every other frame at different locations (hotspots, 180 degree turns, zooms, further along a control spline, etc) to determine texture visibility and mip levels, and communicating the results to an "L2 Cache", to solve some of the texture streaming issues. Not necessarily simple, but nonetheless elegant in its directness.