well, on the plus side, we're about to find that out quite accurately for some sites. Which will be good because it'll help me prioritise the +1 items on my list.
When autocomplete came in I worked out logged in users to probably being about 3% (or something really low) of all internet users in the UK, based on inaccurate figures, gmail usage and some sheer guesswork. Possibly more people stay logged in now because of +1? Doubt it.
Even 3% is a pain in the arse though - especially when you're mid way through developing long tail traffic on a site. Webmaster centrals 1000 to aggregated results won't really go that far for a lot of sites