Faces from Aperture synced over OS X and iPhone in their Contacts?
I do not know of a built-in solution. The Faces feature allows multiple photos to be associated with the same person and thus requires the user to choose which face to use for each contact.
You can use the (paid) third party app Ensoul Contacts from the Mac App Store to achieve this (check recent reviews and decide):

Easily match Faces with your Contacts and enjoy bigger and more beautiful Contacts Photos on your iPhone.
Quickly assign large and beautiful personalized images to all your Contacts from a Mac instead of boring one-by-one image matching through your iPhone's address book interface.
Ensoul Contacts boasts a unique and powerful AutoMatch Faces feature, which allows you to associate iPhoto or Aperture Faces with your Contacts in a click of a button.
Ensoul Contacts allows you to easily navigate through iPhoto, Aperture and Photo Booth libraries, as well as use images from any locations available on your Mac.
In just seconds, transform ordinary snapshots into the contact images of your dreams. Crop, zoom, rotate and modify the colors and textures of your pictures in a variety of ways!
As soon as you set a new contact photo, it gets automatically updated within your Contacts application and is instantly pushed to your iPhone by means of iCloud sync.

See: Ensoul Contacts on Mac App Store