The main highlights from today's announcement: Multitasking, folders for organising apps, enhanced mail, iBooks for iPhone, increased enterprise friendliness, Game Kit, and iAd.
See Engadget's excellent live blog or watch the whole thing yourself. (Tip of the hat to Augustus Chang for the recommendation.)
Macworld has an excellent look at how Apple is doing multitasking without killing the battery.
The 4.0 SDK is now available. Click here to get workarounds if you have problems downloading it.