In "normal" cutscenes in TW2 (you speaking with the blacksmith, a guard, a villager, maybe Iorveth, etc.) it worked fine. Hit the ESC key and the game would pause; hit the same key and it would resume. I can't recall ever having paused during a cutscene with custom soundtracks playing as Finnway said though, so be careful with that in TW3; don't know if that problem still exists.
However, pausing did not work with a controller. Or at least, not entirely. To pause, you had to hit the start button. But to resume the game, you could only press "A" on the "resume game" option. Because "A" was also the button with which you could skip lines of dialogue during almost every cutscene, resuming the game would automatically skip a line of dialogue. As you might imagine, that was real problematic when an important piece of information/dialogue was the one skipped.
So, since we don't know if that's still a problem in TW3, I'd suggest testing it early in the game when a less important cutscene is playing. If you play on a controller, that is.