Just an update to my previous post:
2007 STI's have ports on *BOTH* heads. However, the passenger side head has a block-off plate on it from the factory, as displayed in the picture below. I found this on my 2007 STI, and on my friend's '07 as well. I have also confirmed this from other 2007 STI pictures that I've found on here and IWSTI.
2007 STI passenger side head with factory block-off plate: