Apple renamed the iPhone Operating System to iOS4 for the fourth version. This is because it runs the iPhone, iPod, and iPad, not just the iPhone.
I have access to the pre-release version as a developer and am running it on my phone now. Barricade and X-Ray Timer work the same as always so don’t be scared to upgrade.
The update is available to everyone for free June 21’st and I recommend it very much.
One BIG change is that the update is now free for iPod Touch users! I have spoken to iPod Touch users that couldn’t run the apps because they didn’t pay for the OS update. Now it is free so I recommend to do so. (only generation 2 & 3 can run the update)