I\'ve just installed Magento Community Edition ver 1.8.0.0 (default settings).
System -> Configuration -> Sales -> Checkout -> Checkout Options
What I actually did in the end was compare the new changes with the old code and I discovered the add to cart button was set to type="button" and was not submitting.
Changing the button to simply type="submit" and no other changes actually made it work this is in [theme]/template/catalog/product/view/addtocart
may have to move from base folder if non-existent