Depending on the revision of the logic board, there are two small pads near the bottom of the curve in the board for the fan, that will either be labelled "PWR BTN" or have the Power Symbol next to them. Just short them and the board starts up. Or, remove the RTC battery and as soon as power is applied to the DC-in magsafe jack, the board will start up (but complain about having the time wrong)

This took me AGES to find, because they are obscured with the fan power cable in place. I'm also doing a mod based on a MacBook logic board, so feel free to ask if you have any other questions about it.