Has anyone done up a checkout template where they’ve implemented a checkbox that allows the user to say whether the billing and shipping information is the same?

I haven’t done it but it should be quite straightforward to do using Javascript. I’d opt for using the jQuery library.
Then, have the script auto complete the fields for you when the user ticks the box (and clear them if they untick it).

I can see a potential problem there if they tick the box then update some of the info so perhaps you would also need to disable the shipping address fields when the box is ticked.

You might even want to simply do the auto population of the shipping fields only when the form is submitted - this would also cover things if they changed the billing address AFTER ticking the box.

Phew!

Hmm, ok. Will take a look at the Javascript angle. I can certainly see pros and cons for doing/not doing. It’s not a show stopper though.

Cheers
Brendan

Take a look at this: http://cartthrob.com/docs/tags_detail/checkout_form/#form_fields

That use_billing_info form input will tell the checkout form to use billing info as shipping info. I usually pair this with some javascript to disable the shipping fields.