Well my wife has an iPhone 4 and I have a Samsung Galaxy S. I am not one of those petty fanboys so I feel I can comment in a relatively unbiased fashion about the differences between the two.
Up until recently I felt it was a relatively even contest. I loved that I could run flash on websites, and that I could run multiple tasks simultaneously, my phone was lighter and thinner than any case enclosed iPhone. However, I did envy the iPhone's text rendering, as well as the mute switch on the side.
There was one feature I thought I would be upset to not have and that was the camera flash. The lack of a camera flash has not been that big an issue for me, especially as the iPhone's flash really does give everything a horrible yellow hue, which requires post photo filters before the picture is actually usable, in most situations the cameras are equal.
However, Samsung recently released Android 2.3.3 on the Galaxy S and this update is amazing. The biggest new feature is the unbelievable battery life. As I type my phone is on 58% battery, how long did it take to reach this level, 48 hours! Prior to the update I would need to charge my phone every night with the phone reaching ~10% battery after a full days use, but right now it is potentially up for 4+ days, something not managed since my old Sony phone from 6 years ago.
Essentially I cannot see why now someone would choose the the iPhone over the Galaxy S, not only is the Galaxy S generally better, but you do not have to fight iTunes to get something on to the device! Of course the Galaxy S II is out now and the iPhone 5 is no where in sight, if there was genuine perfect information in the market Apple would be looking at a serious loss of sales to Samsung. Fortunately for them this is not the case and the iPhone will continue to out sell superior devices...