I'm not against the idea at all, but these things always start out meaning well using nice terms like metrics and having extremely valuable uses; like delivering an all round better experience, or freeing up devs for better features and so on. However, with the current feeling in the world today this is going to be a much bigger deal than you might originally think, especially considering the type of individual who ends up using a distro such as this—meaning smart not shady. To my mind it absolutely has to be opt-in, and extremely well advertised to the users. That way you send a very clear message of intent, and avoid the situation where user-x suddenly finds out one day, that his favourite distro—the last place he thought was sacred—started spying on him six months ago without his consent. I think in that situation all those extra stats that you gained would walk away with a bad taste in their mouth. After all opt-io is a metric in and of itself.
It should also be said that that information is far from being anonymous. It is always the same idea: the information sent to microsoft is anonymised, but completely trivial to fingerprint in a heartbeat. One bug report or forum post including emerge --info and I have little doubt any one of us who loves to configure so much would be uided.
I'm not intentionally trolling the idea, I'm highly aware of how important statistics are, but yes I realise I am trolling somewhat just the same :P So what's next? Why not go with the flow and move to systemd? ...and now that I think of it, binary packages would make things a little smoother... :D