I usually print out my hotel confirmation email before I leave to bring along, but often I forget, especially when I'm on a biz trip and transportation to my hotel has been prearranged. Really, no one cares. I'd wouldn't suggest admitting to not knowing, but instead just write something -- anything -- down. No customs official knows the name of every hotel in the nearby city (pick an obvious one... "Tokyo Marriott" or whatever). If you're asked about the address, you can just say you're not sure, but you have a car waiting to pick you up.

Now, this is from experience as a US citizen traveling to Asia and Europe. I have no idea if US border control verifies the name and address of your hotel before letting you through.