Well, the Bible does state that it is the Word of God, instruction upon all mankind. I suspect that it would be most astute to also consider that the language used in Genesis is very descriptive - nothing in it implies a metaphor or allegory. Its furthermore a story that establishes the original sin and the fall of man, a notion which is further used to justify the need for salvation through Christ. We are all sinners after falling from God's grace, and we need Christ to get back to Him; were the Genesis merely allegorical, need for salvation would be severely diminished.
I think its meant to be taken as literal, and even if it isn't, all major traditions seem to take it literally - at least historically.