I think the easiest way would be to use the out of stock feature....then an image appears that tells the customer the item is OUT OF STOCK....but you can change that image to say "item on order" or something.
ON one of my sites I have done something similar....if you go to the following page
http://outdoorcookingdepot.com/index...&keyword=grill
You will see the first item is out of stock...click on that....and you can see an orange highlighted area that shows its out of stock and when it will arrive....
I was allowing them to check out in the past on back ordered items but it became a pain b/c the manufacturers were often giving me wrong dates.....
Is this close to what you are looking for?