Birthmarking is usually a portion of the leaf turning. Since the green is on the older leaves, it may be a natural change in coloration as the leaf ages or it may your new leaves reacting to the change in conditions.

Linda

I havent had it long enough for it to put out any significant amount of new growth.

It may be the former. Has anyone seen an AV with leaves that start out red, and become green?

This is a view with red veining visible through the leaves. It is not on the all-green leaves.

This sometimes happens to my variegated plants if they are in stronger light than they are used to--the variegation turns pink, which is really just the backs of the leaves turning red in response to the light. I don't know if it would happen to a solid green plant or not, but I suppose it's possible.