I'm looking for a good technique to take points, which mark the locations of fence post holes, from a Google map layer to then precisely mark with paint where to drill the holes. I noticed that my neighbor's fence isn't straight, and I want mine to be geographically straight. :-) I'm looking for a simple device or iPhone app to find the points I have for the fence boundaries out in the field.
Google Earth not accurate enough (you are looking for cm accuracy here) especially if you are taking your points from the rubber-sheeted aerial images. When you combine this with the inaccuracy of your phone's GPS (+/- a few meters - the US government quotes about 5m - see here) you will get a wiggly fence line using the method you suggest (or all your posts offset potentially on somebody else's land). The GPS systems surveyors use are a different thing altogether. They also use lasers instead of string. They are much more expensive and more accurate too.
The two suggestions in the comments are worth considering. If you have a smaller or very simple boundary and can clearly identify the corners, go with DPSSpatial's suggestion of string (but not in a high wind as it will bend). If your boundary is larger and/or more complicated, get a surveyor to mark it out. If there are any legal considerations, then the absolute cheapest option in the long run is to have a surveyor come on site and mark the line for you (it will potentially save you thousands in expensive court disputes). They say good fences make good neighbours, but putting a fence on the neighbour's land will make for a very sour relationship no matter how beautifully straight you manage to make it!