I've done 3 AG's now and each one has had a foam appear just before the boil starts.
The first brew I tried to skim it off, but didn't particularly have any equipment to do so, so skimming with a rubberised spatula was pretty in effective.
On the 2nd and 3rd brew, i knocked it back, just to stop it bubbling over.
What i did find with 2 & 3 is that once the boil starts it breaks up within 30 seconds.
On brew 4, i'm going to attempt to skim using a fine seive.