23% of users already on 3.1 software, but 26% still on 2.x edition.
The iPhone OS user-base remains somewhat fragmented, according to stats released by mobile advertising firm AdMob.
According to requests on its ad network, 23% of iPhone OS users - which includes iPhones but also iPod touches - are using the latest 3.1 software, which was released earlier this month.
51% are still on the 3.0 software, but perhaps more worrying for developers are the 26% who are still on 2.x software or below.
Map those stats onto Apple's own sales figures of 50 million iPhone and iPod touch sales, and it means as many as 13 million people who are unable to make full use of apps that take advantage of some of the new features in the iPhone 3.0 such as in-app payments and push notifications.
However, it's the iPod touch users that are slow to upgrade, due to the fact that they have to pay to go from 2.x to 3.x software.
AdMob says that 91% of the requests hitting its network from iPhones are from devices running 3.0 or 3.1 software, compared to only 35% of iPod touch requests.
Apple recently halved the price of the upgrade for iPod touch owners, which may have helped boost the number of upgraders.
On the bright side, if 91% of the 30 million iPhones are running 3.x software, that's 27.3 million people who are ripe for developers' iPhone 3.0 innovation. The dilemma is how to make the most of this without leaving the upgrade refuseniks behind.