I think that in May one can compare YoY units and get reasonable results, at least considering the source. And like Kosty said, the ratios between titles are going to be about the same after adjusting the initial number for the order of magnitude. Once you do that, one can reasonably use the Nielsen ratios on the DVD units.

Here are some deviation stats for those weeks I took the time to calculate. The percentage is the standard deviation from the average between titles, The-Numbers ratios vs. Nielsen. The average for those weeks is only 2.8%, so it's reasonably close. I suspect that the more recent weeks would be on average even closer than that.

Code:

10/23/11 0.40%
10/16/11 0.88%
10/9/11 0.71%
1.76
10/2/11 2.45%
9/25/11 0.86%
9/18/11 6.98%
9/11/11 1.35%
9/4/11 1.04%
8/28/11 1.71%
8/21/11 1.36%
8/14/11 1.95%
8/7/11 8.64%
7/31/11 1.98%
7/24/11 1.34%
7/17/11 18.34%
7/10/11 1.00%
1.85
7/3/11 0.98%
6/26/11 1.07%
6/19/11 1.11%
6/12/11 2.99%
6/5/11 3.22%
5/29/11 2.20%