According to Sony's R&D senior team leader, Neil Brown, the PS4 will allow you to head to a friends house, log into your account, and play any digital game you own.
Heres the full quote from brown:
“You can visit your friends house you can log into your account and play any game from your digital library, which is good. But how useful is that if it takes half a day to download the game you want to play? With Play As You Download you get much quicker access to at least the first section of the game so you can start playing quicker. So this makes a digital library a practical option in the real world.”