Users can sync their iPhone with iTunes and click the check for updates button to get and install the update. According to the Important Information section, it says this version of the software includes bug fixes and supersedes all previous versions.
The download weighs in at 162 MB. I'm downloading it as I type this!
OK - So I downloaded it and I really see no difference. Programs run the same, no new applications, etc. I have never jail broken my iphone. However, I have to wonder why such a large firmware update. It was over 160 MB. The previous 3 were not that big, at least not that I remember them being. I can only assume that Apple has put the SDK in this firmware update - even if hidden. Obviously iTunes will be where we go to download 3rd party apps once they become available. It only makes sense that Apple releases the firmware first, hides it in the root programming, and then within a week or so will update iTunes and that will have the approved 3rd party apps with it. Am I crazy for thinking this? It just sems kind of obvious with no info about the firmware update from Apple. Every other firmware update had detailed explainations as to what the firmware updates did (multi-party text, iclips, etc.) Silence from Apple can only mean that we now have 3rd party support, but we have to wait for them to launch it on iTunes for it to work.