iPhone OS 4 has seven “tent­pole” fea­tures, as Steve Jobs called them today. They are:

• Mul­ti­task­ing: At long last, the iPhone OS gains mul­ti­task­ing sup­port. The rea­son it took so long is to ensure that per­for­mance and bat­tery life didn’t take a hit. Sim­i­larly, the user expe­ri­ence had to match the rest of iPhone OS.
  “If a user has to use a task man­ager to man­age [mul­ti­task­ing], you blew it.” –Steve Jobs
  More than 10,000 apps were looked at to see what fea­tures needed to be avail­able in the back­ground, and from this process they added APIs to allow access to these ser­vices in the back­ground. Using a screen that looks sim­i­lar to the iPhone OS dock, only full-screen, you can choose between any apps that are cur­rently run­ning. Any app can sit in the back­ground and con­sume zero CPU, but be instantly avail­able as soon as it is brought to the fore­ground. No more need to man­u­ally save state for games, for example.
• Fold­ers: Tired of only being able to have 168 apps on your phone at once? Drag-and-drop icons onto each other to cre­ate fold­ers, which are auto­mat­i­cally named based on the cat­e­gory of apps with which you are work­ing (though they can be man­u­ally renamed). Users can have an “unlim­ited” num­ber of fold­ers, though it remains to be seen what that actu­ally means in practice.
• Enhanced Mail: A com­bined inbox for all accounts, threaded mes­sage views, sup­port for mul­ti­ple Exchange accounts, and mail attach­ment launch­ing are all on tap to beef up the default email client.
• iBooks: Apple got into eBooks with the launch of the iPad, and now it’s bring­ing hun­dreds of thou­sands of titles to iPod touch and iPhone as well. Buy once, read any­where (so long as “any­where” is “an iPod touch, iPhone, or iPad”), plus synced book­marks and last-read spots mean that Apple is throw­ing a giant mid­dle fin­ger in the direc­tion of Amazon’s Kin­dle.
• Enter­prise: Apple claims that at least 80 of the For­tune 100 com­pa­nies have iPhones deployed in some man­ner. Remote man­age­ment of mobile devices, sup­port for Exchange Server 2010, and encryp­tion (includ­ing APIs for devel­op­ers to encrypt their own apps’ user-created con­tent) should help move the nee­dle closer to 100%.
• Game Cen­ter (pre­view): Speak­ing of mid­dle fin­gers, Open­Feint and Plus+ have got to be giv­ing Apple the stink­eye right now. Launch­ing with OS 4 as a pre­view, and slated to release this fall, social gam­ing will be a built-in fea­ture. Leader­boards, achieve­ments, and more will be part of the OS just wait­ing for devel­op­ers to add to their games.
• iAd: Don’t cry for Apple, Argentina; despite los­ing an AdMob buy­out to Google (maybe), they did pick up Quat­tro Wire­less, whom Jobs deems “pretty cool”. Regard­less, iAd will offer devel­op­ers an easy Apple way to insert adver­tis­ing into their appli­ca­tions, with full HTML5 sup­port. (Video, inter­ac­tiv­ity, you name it.) Devs get 60% of rev­enues; it remains to be seen where the line will be drawn with what types of ads are accept­able, though if his­tory is any indi­ca­tion, it will be a pretty con­ser­v­a­tive line.

iPhone and iPod touch will be receiv­ing the update this sum­mer, while iPad will see OS 4 this fall. Some fea­tures of the new sys­tem (mul­ti­task­ing is a big exam­ple) will not make it to older devices (pre-3GS and touch 3^rd gen­er­a­tion), with Apple stat­ing the hard­ware sim­ply isn’t there to sup­port it.