If you have been following WWDC 2010, you will be aware that iPhone OS 4 is no longer called that and has been renamed to iOS4.
iOS4 brings a lot of new changes including Multitasking, Folders, Retina display integration, unified inbox and conversation threading, enhanced camera and photos apps and deeper enterprise support.
Now, those features in itself are pretty exciting, however, the good news is that all other than iPhone 4 users, iPhone 3GS, iPhone 3G and iPod touch excluding first generation users will also get a free update to iOS4 on June 21st.
This is definitely good news since iOS4 brings in a host of features which are definitely worth using. So set your calendars then to June 21st and fire up iTunes to get your free update to iOS4.



