Ryan Brenizer shares his thoughts on HDR apps for the iPhone (3Gs). Results are pretty damned good. Check out his post for examples and links to Pro HDR and TruHDR in the iTunes store.
HDR is just a tool to fix the inherent problem most digital cameras have of being able to capture a much smaller range of lights and darks than the human eye, and few cameras need fixing as badly as a tiny cell phone camera. With all of those pixels crammed in to a space so small, each pixel isn't receiving very much light, and that tends to mean noisy images with blown out highlights. The noise problem is hard to fix, but tonal range is relatively simple: Just take a picture exposed for the shadows, another for the highlights, and slap them together. And that, simply and easily, is what both of these applications do.