Its a unique and proprietary technology
id tech 5, yes. But megatextures/virtual texturing is not a proprietary technology and other Studies use the same technology.
Lionhead are working with it, or RedLynx, or as far as I know Dice, too.
Most games use a megatexture for the terrain, but I don't think there are others that do the virtual texturing to the extent that Rage (and future id Tech 5+ games)
Megatexure is fairly new and until now almost no game use it.
In the future very likely, but "most games" most certainly don't use this kind of technology already for terrain.
This engine has a great advantage over others in future technologies when u'll get the HD video on wireless steam and has the game running on a beasty workstation like server which is able to handle the load of the REALTIME 100+ Gb texture and vertex calculations. Than you will have photo realistic graphics on a hand-held console. Not really far away from now.
Megatexture doesn't have any advantage in regards of playing streamed games. The only thing that matters is a steady internet connection.
And 100GB textures wouldn't be a problem for PCs and consoles, you don't need a beasty workstation that's the nice thing about megatextures, but only if most of them had BluRay and fast HDDs...