Insidious Technologies has released a virtual texturing add-on for Unity Pro. According to Insidious, Amplify enables level artists to use large volumes of texture data without worrying about memory limits. Virtual texturing works by slicing large textures into smaller tiles. At runtime, only tiles visible to the camera are streamed from disk.
The add-on supports per-material textures over 4,000 x 4,000 pixels in size, including diffuse, normal and glossiness maps. It is available for Windows and Mac OS X.
More about that including a promo video, you will find here: cgchannel.com