There is now a Nexus 7 Guidebook on Google Play. Starting on page 67 it talks about usb connectivity.
It says that you can connect mice, keyboards and "other" devices with an "adapter". I had assumed this was true but it is comforting to see it in print.
Here is a link to the OTG (On The Go) adapter I think I have - I could have the Motorola version.
For those that are interested I'm attaching a thumbnail of the USB pinouts. As you can see the only difference between the regular and the OTG cable is that pin 4 and 5 are tied together. If the tablet senses a ground on pin 4 it knows that it should act as a host usb port if not then act as a device port. Simple and easy/cheap to implement.
Regards - John