Currently, it's only possible to show the quantity input field on the single product or cart page. It would be great if the quantity field could be displayed on archive/shop pages too.
I agree with Teddd1 - this is an important feature. Other e-commerce plugin offer this out of the box and it's very useful for certain products (i.e. accessoires - a single product page with product description, etc. doesn't make much sense here - or if you try to sell seats, tickets, etc.). This could be a premium addon too.
This can be achieved by customising the template files now (templates/loop/add-to-cart.php) - I would think it would only be applicable to simple products. A form would need adding to that file (with quantity selector) and it would need styling.
Adding this to core would be too large a burden on theme and plugin developers IMO (extra elements to style, more things to consider with extensions such as addons). I'd like to keep the core simplistic and consistant across product types when possible.
Note, not ruled out as an extension idea. could post on ideas.woothemes.com to judge demand.
You can vote on this feature suggestion here: http://ideas.woothemes.com/forums/133476-woocommerce/suggestions/2499031-add-quantity-to-shop-page