Get-rich tip: Be the first grocery store to provide a digital purchase history to customers, for use in household planning.
This would actually convince me to use a frequent-shopper card and provide accurate information for it. Every time I made a purchase, a record (in some open standard format) would be emailed to an address of my choosing. In this fashion, I could build a household food database.
A sufficiently sophisticated grocery chain could even make the purchase history available via Web app, though the email option should still be there so other tools could be used.
An ambitious customer could even use a handheld scanner at home on items as they are thrown away. This would allow you to have an accurate, available-from-anywhere database of what is in your fridge and/or pantry. A scanner would also allow you to scan in purchases from other retailers (i.e. wine from your local wine shop, cheese from the cheese shop, etc).
And of course you could enter data in manually. With the right interface, this wouldn't be such a pain. With one click, for example, you could record the repurchasing of anything you've ever bought before.
If a bar-code scanner were built into an iPhone or Blackberry, one could quickly check whether a particular item is already in the cupboard at home, or even find out if one is being charged a reasonable price.