How do I determine what is waking my machine up?
You can find a lot of information in the system log.

Open the Console utility and this will allow you to easily search the system log for "sleep" and "wake".

The information is not easy to understand but if you do find some lines in the system log you can't understand feel free to update your question.

EHC1 is usually something to do with a USB device. Do you have an external drive attached to the machine? Is it powered via USB?