Someone needs to create a Delicious Monster spin-off for food, where you enter in the items you have in your pantry and the items you buy at the grocery store. You could even scan things in via iSight when you bring them home. Using this database, you could create recipes based on your available ingredients, and also incorporate the date you purchased the items to adjust the recipes based on the amount you have and the oft-suprising expiration dates.
The iPod syncing feature would also be really handy while grocery shopping. Not only could you bring your shopping list digitally (sorry, Bill, no more books to write), but also know what ingredients you already have at home if you come up with a new meal idea from things you spontaneously purchase at the store.
I use MacGourmet as a recipe manager and it seems to do about half of what you want, including the iPod syncing.
I wonder if RFID would work better than barcode scanning. One of the few good things that Wal-Mart has done is to force a lot of their suppliers to begin RFID tagging items, if you had a scanner in your pantry or fridge that just knew what was in there, that’d be better than having to scan everything in manually. Sensors in the container could even update the amount of, say, milk that you’ve got left in that half gallon. When things get low, it could automatically generate a shopping list — hook *that* up to an Amazon web service and you’ve got something magic!
I use a ingredient-based cooking pattern as well (as opposed to a recipe-based pattern). To help I convert my recipes into separate documents and then use Punakea (http://www.nudgenudge.eu/punakea) to apply tags of ingredients to the file. When I want to cook, I check what I have fresh or still in the fridge, fire up Punakea, and apply tags to retrieve recipes based on available ingredients. For someone like me who has ready access to fresh food, getting recipes based on ingredients I have is much more important than an arbitrary databased of recipes.
The ideal web app would have an AJAX-based live search to narrow down recipes as ingredients are added or widen the search when ingredients are removed.