I saw a (I believe) 60 minutes story about WalMart a while ago and they stated they stored everything at line-item resolution for 2 years of sales. With their volume it's an amazing amount of data.
UPDATE - did some quick googling and that was 10 years ago. I have no idea how big their DW is now (was 583 TB then)
The one thing about Walmart though, and many of the largest retailers like Kroger, is that that market basket data is far too valuable to allow it into the hands of anyone else - and they won't/don't sell it.
Other smaller retailers, including Target and hundreds of others, will pool their transaction data into blind coops like Abacus, or syndicate it via companies like dunnhumby.
