LR4 Beta & DNG (found my answer)

There is an optional "lossy compression" option when converting to DNG in Lr 4 Beta. For raw files, this keeps the data in scene-referred (linear) state, but uses JPEG compression (instead of lossless compression or uncompressed) for storage of the image values. Since the image data is scene-referred, you still have access to all the normal raw-based goodies, like changing WB, doing highlight recovery, noise reduction, sharpening, lens corrections, etc.

However, the files will be substantially smaller in size (e.g., 4 MB for lossy compression, vs 13 MB for lossless compression). Also, when choosing lossy compression you also have the option to save a reduced-resolution version of the image data. For example, your original image may be 24 MP, but you can save a DNG that is only 10 MP, or screen-res (say, 2000 pixels on the long side).

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Eric Chan
http://people.csail.mit.edu/ericchan/photos/
http://people.csail.mit.edu/ericchan/dp/Epson3800/index.html