AdMob, a company that supplies advertising services for iPhone developers, says that between September 15th and 22nd only 23% of devices were updated to iPhone OS 3.1. This counts only devices that run apps with AdMob ads.
Some more statistics: 51% are still running OS 3.0, and 19% are still running v2.2.1, while 7% are still running v2.0 or earlier.
The reasons, as always, are very different. The update for the iPod Touch costs money, there are problems with internet tethering in 3.1, there is no firmware 3.1 jailbreak tool for Windows and it is impossible to jailbreak iPhone 3GS.
Have you perfomed the upgrade or not? Please, leave a comment why.