0.4% are running the newest version
Here's the rundown:
Android 2.3 - 0.4%
Android 2.2 - 51.8%
Android 2.1 - 35.2%
Android 1.6 - 7.9%
Android 1.5 - 4.7%
< Android 1.5 - 0.0%
To date, only two phones have Android 2.3 available - the Nexus One and the Nexus S. The former is the phone developed internally by Google, which sold terribly, and the latter is the most recent Android phone that hasn't been on the market long enough to reach serious penetration levels.
However, 35.2% of Android owners stuck with an older version is still a pretty high number. What's worse are allegations that Samsung has been blocking mobile carriers from releasing over-the-air updates to Android 2.2, because it wants owners of its phones to have to buy a new handset to get the better version of Android.
Samsung hasn't admitted it is doing that, but it hasn't denied the allegations either.
Nevertheless, eyes are on Android 2.3 now, and no one is talking about when other phones might get that upgrade. Many are still just now getting used to 2.2.