"According to Google’s own numbers, almost 90% of Android devices worldwide are still two or more versions behind. The vast majority are running a version called Gingerbread, which came out at the end of 2010. It’s taken almost eight months for the last version of Android, called Ice Cream Sandwich, to reach 10% of Android devices."
Sidenote: This is one of the issues that prevents me from considering an Android phone. Say what you will about the iPhone: When an update comes out, it comes out for at least the current and last gen devices, and you can install it whenever you want.