The webstorage plugin is even a part of C2's install for several versions now, you don't need to download it from the forum thread.
The post is useful as documentation though.
Also, even on Iphone it will still work in the browser, so the datas should still get saved into the browser and the plugin should work.
I can't say for sure that it works, since I don't have an iphone or smartphone to test it on. If you do, your best shot is to give it a try.
Another way would be to send the datas via AJAX to a website that would save them in its database. AJAX tutorial - related topic