I've never been happy with the old 'iTunes' model ("flip-flop" buttons; see this question) where the play and pause are the same button, with an icon showing what state you can go to (Play when you're paused, Pause when you're playing). Users often mistake action for state or vice versa.
Lately I've been seeing buttons marked
> / || which have two states, unhighlighted and highlighted. Users seem able to grok this, though I'm never sure if 'in' means 'go' or 'pause. It's vaguely unsatisfying.
I was debating a single pause icon:
|| which gets pressed 'in' when engaged, like an old tape deck; but perhaps that too would be confusing.
Can you think of another way to present this play/pause dual?