CNet says that Apple improved the camera on the iPhone 3G, and judging by my own use and putting both camera side-by-side, they're definitely right. Why would Apple include a bigger lenses for nothing? Here's a comparison from CNet:
First iPhone:
iPhone 3G:
When I compared them myself, I got the same result. Clearer, more vivid pictures with less noise on the iPhone 3G. They both have great cameras, but it's nice to know that Apple improved everything on the iPhone 3G. Another nice thing is that the clearer camera means that you can zoom in a bit more on the iPhone 3G without the image starting to look worse.
First iPhone:

iPhone 3G:

When I compared them myself, I got the same result. Clearer, more vivid pictures with less noise on the iPhone 3G. They both have great cameras, but it's nice to know that Apple improved everything on the iPhone 3G. Another nice thing is that the clearer camera means that you can zoom in a bit more on the iPhone 3G without the image starting to look worse.