I can tell you the selection of Wii games at most Chicago area Blockbusters are limited. I found one store that has no games, one that has 2, and another that has 7, with 6 of them checked out at the time.
I also hear similar stories with GameFly etc. places. That the selection is limited and there is often a queue for their Wii games.
I think the reason is that there is not many systems out to actually make a profit from the rentals, even at 6.99 for 5 days like at BB. They would have to rent the same game out about 10 times to make a logical profit (not including business expense figures).
I am estimating that my area sold about 1-2k Wiis. So about 1 to 2% of all Wii owners would have to rent that individual game to keep renting that game profitable.