Prior to the June 24 launch of the iPhone 4, iPhone 3G, 3GS, and, recent iPod touches can update to iPhone OS4.
iPhone OS4 offers seven multitasking services, which will allow tasks to be performed in the background while preserving performance and battery life. Users can make Voice over IP call while playing a game or checking email or listening to music.
Update 6/28/10: Downloaders of the iOS4 update reported that it really drains battery life, reducing battery-use times by as much as 50%.
The new organization tool is folders for holding information. Email
features include a unified inbox fro switching between inboxes,
organizing messages by threads, and opening attachments in 3rd party
apps.
iPhone OS4 includes iBooks to browse, buy, and read books on a
mobile device. Users can read books while listening tor music
collection. Books can be browsed by title, author, or genre.
Advertisers can opt for the more media-rich iAds, a potential source of
revenue for developers. iAds will act like applets instead of taking
clickers away to the web. Gamers can social-network in the Game Center
that offers gamer-matchmaking, leaderboards, game achievements, and
friend-tracking.
Developers are given more access to edit events in the Calendar and
create SMS messages. Developers can develop apps that for photos and
video playing/capture.