if ATOM GPS is the windows client to tag your photos, you will still need a GPS log trace.
Actually we bought from VSO software an iPhone app we will maintain, this app is GPS Stone.

GPS Stone works in iPhone and record your GPS position then export the trip as a .GPX ( it contains gps position and timing )

you can watch the .GPX with ATOM GPS or with Google Earth

Once you have your GPX file loaded, you can associate ANY digital camera or photo with a simple rule, it will tag the GPS position according the time and date you shoot the picture.

the absolut reliability is the distance from your digital camera and the iPhone or GPS Data logger

it works also with several digital camera, so it is much better solution than a GPS flash shoe

the problem will be the autonomy of the iPhone battery, but if you are using it during a road trip, it shouldn't be a problem.

Download GPS Stone for iPhone

as usual , if it is a topic of interest, just post your question and I will do custom article for you.