Pricecheckah [App Store] is US$4.99 At the set up screen you can choose online retailers like Amazon or Buy.com, and brick and mortar retailers like Target or Walmart. You can also add your own. I added Best Buy by just typing bestbuy.com.
I typed in a UPC code and the app made several suggestions and did find some better prices. Of course you could do this from any browser using a site like Froogle. This app, of course, is meant to be portable, but you might not be very popular at some retailers doing price comparisons. On the other hand, unless you are in a hurry to buy something, you will likely find a better price online.
Some things I don't like: The app wants my email address and a password. I'm not wild about giving someone my email when I'm not sure what they are going to do with it. Sometimes the app is very slow; I touched the settings button and it took several seconds to get to the settings screen. It didn't happen every time, but enough times to mention it. It also locked up a couple of times and said it couldn't find a network when WiFi was on, and my wireless router was right next to my iPhone.
I also think the app is pricey at US$4.99. There is a similar app called Checkout Smartshop [App Store] which is the same price but has many more negative reviews. The developers promised a bar code scanner and it hasn't arrived. We've also looked at RedLaser [App Store], which does have a barcode scanner. It worked OK for me, but many people are having problems with it. It's US$1.99.
In summary, it doesn't look like the perfect shopping app is here, but hopefully developers will keep trying. Pricecheckah works on the iPhone or iPod touch, and checks prices in either the US or the UK.