Only just found this out while working on shadows for my game, the ADD ink effect is fine on both my 2g and 4g ipod touches but SUBTRACT only works on the more modern 4g model, doesn't seem to be a memory error but Xcode spits out a line access problem:

"glBlendEquatonSeparate(equationA, equationB);"

Is this just a bug or does subtract not work on older devices? wound be nice to know thanks