I have repeated Andrew's test with the same results. I also included a test for Sharpening Screen High and Low, and found that the differences were more noticeable when using Blend Mode Difference rather than Subtract.
My tests covered both the extreme ends of the range (1 ppi vs. 65000 ppi), as well as tests within the 70-120 ppi range mentioned by Jan. To ensure image content wasn't implicated in the null results I also ran the test with Martin Evening's test image (Lightroom 4 Book, page 358, Figure 6.7).
Cheers,
Paul