will everything developed in 1.1 now have "slightly" more luminance noise in 1.2?
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I don't know about "everything" but the implication of the Release Notes is that if you have a RAW image whose Luminance Noise setting is 0, and you convert it twice -- once with LR 1.1 and once with LR 1.2, the LR 1.2 will exhibit slightly more luma noise.
I just took three test images, one at ISO 800, two at ISO 1600 (canon 5D and 1D II images) and did a 3-way test ... ACR 3.7 in CS 2, ACR 4.1 in CS 3, and ACR 4.2 in CS 3. Same RAW converter settings for all three. Luma set to 0, all sharpening disabled.
The differences between 3.7 and 4.x in noise are pretty clear. 4.x exhibits clearly less noise, but leaves a more discernable visible structure which has been the topic of much discussion among some users. The differences between 4.1 and 4.2 (and for LR, 1.1 and 1.2) that I saw were very, very slight to my eye. There is a bit more luma noise in 4.2. Frankly I doubt I could see a difference in the print. It also depends on how sharpening is applied.
YMMV ...