Yes, you can lay the PS5 console on its side. Long answer: Sony may have led with the vertical arrangement for the PS5 reveal, but don't fret: you can place your PS5 horizontally, as with all previous PlayStation consoles. Sony has confirmed as much in a couple of images:
As you can see, the PS5 can indeed be put on its side with the use of a stand to accommodate its curved surface. The system comes with a base that allows you to configure it both vertically and horizontally, so you can display however you prefer.