I posted a quote from the Netflix "Terms of Use" in another thread - it seems to suggest that Netflix may delay shipments by a day and send lower priority items to high-volume renters.
There also seems to be a lot of unofficial speculation out there about Netflix "throttling". Here are various links I've found, or do a Google search on "Netflix throttling".
Hacking Netflix site
ManuelsWeb Netflix journal
An ABC San Francisco story
If Netflix is getting 1-day service to a high percentage of customers, they are certainly no strangers to the "high-volume" problem. Frankly, I'd rather a company be open about how they deal with it (even if ZIP's approach may not be ideal) rather than have them do things behind the scenes.