Hello.
Does one exist?
Search engines find a number of them, even if a number of people here prefers free stuff to open source. So the reply is "yes", unless you make a sharper question.
If not, is anyone aware of any attempts to develop one?
I know about the "f-watch" (f can be a four letter word, like "free"), that was on main track at fosdem one month ago. Not often found as "open source smartwatch" by search engines, so worth noting here.
http://www.ohwr.org/projects/f-watch/
But, again, what is your point?
/alessandro
(Sorry for looking rude, it's not my intention but I know it may look like that.)