My previous 2 companies it rarely happened, if ever. But it has happened a lot with me at Google. But I also think it's a property of the teams I am on. Other teams it doesn't seem to happen as often.

Having whiteboards around, I will see people talking about certain things or explaining some new system, and it's easy to jump in. Maybe these could be considered impromptu meetings, but they still have been great for me.

Agreed -- I haven't worked on a project in the last ten years that didn't have some significant portion of relevant staff remotely located.

I'm not a developer, but I don't work with a group of people located in the same building, so all projects are de facto pretty remote heavy.