iPhones in iPads are a great example. There is no mouse, there is no clicking (only tapping), there is no rollover or hover. Lots of great interactive designs assume that users will use their mouse to interact with the page. Remove the mouse from the equation and certain things that used to be exciting now fail to work.
Are your designs “finger friendly”? Do they work just as well by tapping as by mousing? Do your pages resize– when one might reasonably expect it– to fit the width of the device? If you display a phone number, can you tap it to dial it?
If you aren't accommodating these devices, at best you are diminishing your brand experience, at worst you are failing a forward thinking portion of your audience.