Sidestepping (widely) around the issue of common Wal Mart customer's life choices, I've been somewhat interested to know how well Vudu is/will do on WalMart.com.
All joking aside about Wal Mart folk, I've always wondered if Vudu's 3 tiers of quality was just too confusing for most people that just want to click and play. AmazonVOD (by comparison) just has SD and HD, which most people probably feel they understand (even if they have been mislead by media companies) enough to decide which they want (read: Me wants HD, always) but HDX's somewhat ambiguous naming, along with the relatively high bandwidth requirements, would seem to be a recipe for confusion.
Netflix does it best imo as far as point of sale (that is if they offered point of sale transactions.) Pick the title and get the best quality your connection/hardware will give you.
-Suntan