iPhone OS 4.0 released Monday Apr 12th
iPhone OS 4 was announced by Apple on the 8th April. Apple state the new OS has over 100 new features, but the one that people will be excited about are the arrival of multitasking (of a sort) on the device.
In addition to multitasking Apple have announced improvements to the mail app, the arrival of the iBook store (in the US only so far) and the ability to vreate folders to organise apps.
Apples own OS 4 page is available here: http://www.apple.com/iphone/preview-iphone-os/
