5.1, but the minor bump in version number doesn't tell the whole story of the speed increases you can expect if you aren't one of the 25 million+ who have already upgraded . New Relic timed the loading speed for iOS 4 and iOS 5 devices in tests with 3,000 web apps and compared the average loading times for each.
According to the tests, iOS 5 and Safari 5.1 isn't just faster, but a lot faster than its predecessor. In the tests, iOS 4 needed an average of 9.6 seconds to load the web apps, but iOS 5 only needed 4.1 seconds. And, in a controlled test it took the iOS 4 phone (presumably the iPhone 4) 6.34 seconds to load a web app, whereas it took the iOS 5 phone just 1.88 seconds to load the page.
Of course, those of you who have already updated your iPhones or iPads to the newest version of iOS may have already noticed the increased browsing speed, but for the rest of you: the proof is in the numbers. Now, stop being lazy and update your software!