It's a good idea in theory, but this is what will probably happen:
The Linux/xBSD geeks will LOVE this idea... until they see the hardware specifications on these systems. Once they find out that they're filled with a bunch of intergrated crap that has lousy driver support, they'll probably to back to buying Dell's or building their own budget systems.
Joe Six pack won't understand what "Operating System not Included" means, and buy one anyway. Once he plugs it in and finds that it won't boot, he'll probably bitch to Walmart saying that "it's broken". Then, he'll either return it, pay $100 for a copy of Windows, or have someone install a pirated copy for him. A few of these people might try Linux, but most of them will give up on it when they find out that it won't run TurboTax, or Nascar 2002. Remember, we're talking WalMart shoppers here, not CompSci professors from MIT.
Cheapskates who know what they're doing might buy them anyway, however. They already have their pirated copies of Windows XP Pro Corporate on CD-R, and won't mind saving $100 by installing an OS on their own. Microsoft will catch on to this, sue some WalMart customers on trumped up piracy charges, and Walmart will be forced to pull the entire line to avoid the bad press.
Of course, I could be wrong, as I'm not a fortune teller. You don't have to be Miss Cleo to see some trouble in Walmart's future from these computers.