To whom it may concern...
I was having issues with fonts that I created using Hiero...the png would look correct, but when displaying the fonts on the iPhone (or simulator) they showed odd lines at the edges of some of the characters.
I eventually realized that the edge of the next character in the atlas is getting picked up. For example, in the png we have "ABCD..." when displaying the "A" using a BitmapFontAtlas I would be seeing the left edge of the "B" (1 pixels worth).
To solve it: I opened the .fnt file in a text editor and decremented all the "width=" values and all the "height=" values by 1. In other words I replaced all "width=16" with "width=15" and so on (working from lower values to higher, so as to not change any values twice).
Now my fonts look perfect!
I have a feeling this only applies to certain fonts, at certain sizes, but I'm not sure. I thought I'd throw this out there in case anyone else was having this same issue, this is an easy solution.