According to Mrgan's Twitter page, the app existed as early as May 31. That's either tremendous foresight by Mrgan, amazing luck that Apple announced Safari as the official iPhone app engine yesterday at WWDC, or some combination of the two.
The app itself is straightforward: you pick from basic categories such as "Fruit," "Veggies," "Meat and Seafood," and "Drinks," which then brings up a list of more specific items. Click an item, and it's added to your shopping list. Not mind-boggling, for sure, but it is extremely useful.
The iPhone itself won't be released until June 29, but OneTrip can save your shopping list as a cookie. Thanks to that, you can start prepping for your iPhone's first trip to the grocery store right now...if you can afford groceries, of course, after buying an iPhone.