[watchit]

If you want to keep the client contact info in Address Book... rather than spread through OF and later, OF's non-Spotlight searched Archive... then you can drag the client's AB vCard to the Action or the Project for quick access. But this is only viable if you keep it on one machine. The link between AB & OF will not work on other devices you OF sync to such as iPhone or networked computer, even if you use MobileMe to sync AB.
It would go some way to filling the void on Mac of a lack of a decent PIM if OF could have a robust system of linking AB contacts that could survive sync.