Genesis 3:15 is not the protoevangelium

Literal Meaning

The context describes God's judgments upon Adam, Eve, and the serpent for their sins, all of which appear to be quite literal:

• Verse 14: The serpent is cursed, will crawl on its belly and "eat" the dust
• Verse 16: The woman's labor pains will be increased and will desire to dominate her husband, who will instead dominate her.
• Verse 15: The serpent's offspring and the woman's offspring will be hostile toward one another, with the woman's offspring attacking his head, and he attacking her offspring's heel.

Verses 17, 18, and 19 each seem to describe very literal punishments, as well. If we take all the other judgments as being literal, why pick a single one out and say it is something more?

The word "offspring"

The word translated "offspring" or "seed" is the Hebrew word zera. This word is a collective singular, meaning it refers to a group. It does not refer to an individual, as we would expect if it were a reference to Christ.

The meaning of strike/bruise/crush/attack

1. The same word--sup--is used to describe both the action of the serpent and the action of the woman's offspring. It is generally used to refer to a non-fatal attack. There is no reason to interpret it as meaning a mere wound (temporary death) when applied to Christ, and then as a fatal blow (ultimate defeat) when applied to Satan.
2. The grammatical structure of the verse suggests an ongoing struggle rather than a one-time defeat of the serpent.
3. The woman's offspring's attack on the serpent is mentioned first. If this were a fatal blow or final defeat, it would be interesting to see how the dead serpent would be able to then attack the woman's offspring.

Hostility

The word translated "hostility" or "enmity" is eba, describing an enemy relationship. This hostile relationship is listed as a punishment for the serpent. Though the relationship between the serpent and the woman will be hostile in both directions, it will be a losing battle for the serpent.

Note also that God says He would put hostility between the serpent and the woman. Since there was already hostility between Christ and Satan (initiated by Satan, no less), the new relationship would not make sense as descriptive of Christ and Satan. The new relationship God was creating was one of hostility between man and snake.

As a matter of clarification, be aware that though the serpent is referred to as the addressee, the serpent's offspring are included in the statement. This corporate solidarity can also be seen in Genesis 28:14Genesis 28:14
14 Your descendants will be like the dust of the earth, and you will spread out to the west, east, north, and south.. In this verse, though Jacob is the addressee, it is Jacob's offspring who will spread out.

Consistency - Satan or the snake?

Reading Genesis 3:15 as a messianic prophecy would require flip-flopping between subjects. That is, it would place some parts of the judgment upon the serpent as an animal while placing other parts upon Satan. If Satan is the one who will attack the woman's heel, then it is also Satan who is cursed "above all the wild beasts and all the living creatures," and it is also Satan who will eat the dust all the days of his life.

What's the significance, then?

Changing one's view of Genesis 3:15 from a powerful prophecy of Christ to a mere statement about man's relationship with snakes might be a real letdown. However, Old Testament Jews would encounter venomous snakes quite often, and knowing the story of Adam and Eve, such encounters would serve as practical reminders of the effects of sin.