Hi Scott.
My view is this:
We can't know for sure, but Google have said that user metrics are a part of the equation (citation needed), so it seems likely that they will use that sort of thing.
I generally assume that they will use any data they have access to.
Now:
As far as I can see, they can't track bounce rate in the literal sense. If someone finds my site in a search result and clicks it, and then immediately "bounces" to say facebook, or just closes their browser or whatever, Google won't know that has happened.
However:
A scenario that Google can track is if the above happens but then the user bounces back to the Google search results. If that happens it seems to me that Google could use that as a signal that the user's query was not entirely answered.
So it's not quite what we would call bounce rate, but it's similar.
In that scenario I think one of three things can happen:
User returns to Google and does same search
User returns to Google and does similar search
User returns to Google and does completely different search
I imagine that Google will take a different meaning from all of those possibilities, and obviously compare your metrics with those of the other sites ranking for that search.
Now, HOW they use those signals is anyones guess, but I think it's a safe assumption that they do, because why wouldn't they.
So my advice to anyone worrying about bounce rate would be:
If a lot of your visitors are bouncing back to Google, fix it.
If your users are bouncing on to other sites, that may not be an SEO problem, although it's potentially a sign that you're doing something wrong, so fix it if you can.
Of course there are examples where a site would be expected to have a high bounce rate, but it's how you compare to your competition that matters!
I'd be interested in other opinions on this though...