Chrome has had SPDY enabled for at least a year now -- since around Chrome 10, I think? And according to Statcounter measurements for this month to date [1], 28.75% of all page views currently come from Chrome 19.
From the same Statcounter numbers, 17.27% of all page views are from Firefox 12. So that's over 46% of page views this month using the latest versions of Chrome and Firefox. And another few percent are just one or two versions behind. So a couple of months from now, 50% of page views measured by Statcounter should come from SPDY-enabled browsers, especially if IE marketshare keeps shrinking rapidly.
Also interesting: according to that graph, as of June 2012, Firefox's major stable release at last surpassed IE's major stable release--just a few months shy of the 10-year anniversary of the release of Firefox 0.1.
EDIT: Ha, actually, drilling back further, looks like that's not true at all: Firefox's major release has surpassed IE9 for a while now. I was misled by IE9's slow adoption curve.
From the same Statcounter numbers, 17.27% of all page views are from Firefox 12. So that's over 46% of page views this month using the latest versions of Chrome and Firefox. And another few percent are just one or two versions behind. So a couple of months from now, 50% of page views measured by Statcounter should come from SPDY-enabled browsers, especially if IE marketshare keeps shrinking rapidly.
[1] http://gs.statcounter.com/#browser_version-ww-monthly-201206...