I experimented iphone3G, 3GS, and 4 for scanning doc by taking picture in different devices accordingly.
Both the back camera in iphone 3GS and 4 have good resolution like a flatbed scanner can do.
I used this app (DocScan) for the the experiment and shared the results with you:
1. Auto detect whiteboards → rectify perspective distortion → brighten up
2. Select crop text area → rectify perspective distortion → brighten up
3. Zoom in → Zoom in (with shadow removed)
4. Auto detect business cards → rectify perspective distortion → brighten up
You can try it with different samples by yourself. The app is free to download.
Both the back camera in iphone 3GS and 4 have good resolution like a flatbed scanner can do.
I used this app (DocScan) for the the experiment and shared the results with you:
1. Auto detect whiteboards → rectify perspective distortion → brighten up

2. Select crop text area → rectify perspective distortion → brighten up

3. Zoom in → Zoom in (with shadow removed)

4. Auto detect business cards → rectify perspective distortion → brighten up

You can try it with different samples by yourself. The app is free to download.