Well, that was quick: Analytics firm Localytics says that iOS 5 is already installed on one of every three eligible devices, meaning that just a week or so after release, it's already set up and running on a big chunk of the ecosystem. Even taking the iPhone 4S out of the equation (which of course comes with iOS 5 installed), the number's still up to 31%, which is a pretty solid install base put together so quickly.
As you can see from the chart above, the iPad 2 actually has the highest rate of install, according to the data Localytics is looking at. Makes sense that both the iPad 2 and the iPhone 4 would be up there, as those are likely Apple's most devoted users, dedicated to keeping the latest and greatest Apple products running.
But I think in general this information shows that Apple has done a great job of letting its customers know just how important and useful the upgrade to iOS 5 can be. Most devs will still want to support older versions of the operating system, obviously, but widespread support like this make it easy for most devs to support options like iCloud and iOS 5's other new features.