iOS 5, Apple’s new mobile operating system, was shown off at WWDC. As  expected, the new version includes a brand-new notifications system, and  system-wide social integration. Here’s everything you need to know:
Features
Notifications Center: all current iOS users know  that the current notifications system was showing its age. Apple has  turned that around by building a new system, which unobtrusively  displays all notifications at the top of the screen, even during games,  and on the Lock Screen. By sliding them down, users will be able to  visualize all unread notifications, or dismiss them completely by  hitting an "X" button next to them. Very straight-forward. Notification  can include missed calls, voice mail messages (which can be played right  from the Notifications Center and even notifications from other Apps.
This system is very similar to MobileNotifier, which isn’t surprising, since the company has recently 
hired its main developer.
Reminders: this feature, as the name implies, allows  users to set different reminders for different times of the day, and  different places. Thanks to the geolocation support present in devices,  users can now configure Reminders to pop up a notification at a certain  time and at a certain place, and even sync across devices using iCal.  It’s really to-do lists on steroids!
Enhanced Safari: The iOS browser now looks a lot  more like its desktop counterpart, with a few extra features of its own.  Tabbed browsing now looks identical to the way it looks on the Windows  and Mac versions, with tabs displayed across the top, at least on the  iPad. The new Safari also brings the Reader feature, displaying a  website’s content in an easy text only view, but unlike what happened  before, users are now able to bookmark content onto a "Reading List" for  later or send it to friends.
Twitter Integration: as rumored, the new iOS 5  includes deeper system-wide integration with Twitter. While there’s no  support for other social networks, this feature is integrated with many  apps, including the Camera App, allowing you to quickly tweet out what  you’re working on. You can log into Twitter from System Preferences.  It’s a single log in.
New Mail client: similarly to what happened in Lion,  the iOS mail client is also receiving an upgrade. The App now supports  draggable addresses, as well as the ability to flag messages and search  them and dictionary lookup, by tapping on a word and selecting "Define".
iMessage: it’s like iChat for iOS users. It lets you  send text messages, photos, videos and even have group conversations,  similarly to any other messaging client. This application is also a  great new example of how Notifications Center, since this App interacts  beautifully with it. With this out the gate, I one is left to wonder  what will happen to iChat.
Independence from computers: Previous versions of  iOS required users to connect them to a computer in order to set them  up, update them or sync them. iOS 5 will put an end to all that, by  allowing users to update the device over-the-air. There’s also a new  set-up wizard for new devices, allowing them to be configured for the  first time without the need for a desktop computer. Can those who hate  iTunes finally proclaim victory?
Tens of small additions: iOS 5 includes a number of  small improvements, such as direct game downloads in the Game Center, a  new optional keyboard, enhancements to the Music App on the iPad and an  enhanced Camera app.
Availability
If you’re a developer and have an iPhone 4 or 3GS, a 3rd or 4th  generation iPod touch or a first or second-gen iPad, you’ll be able to  grab iOS 5 from Apple Developer Center today for testing purposes, so  you can get your Apps ready for many of the features we’ve talked about,  such as Notifications Center.
Regular users would be able to grab the software for free later this fall. For more details, check 
this article.
Jailbreak?
Those using a jailbroken device must be warned that no jailbreak  solution is available at the moment. Upgrading to the new software 
will break your jailbreak. You can read more about the state of jailbreaking on an editorial I 
wrote up yesterday.