Apple has released the second beta of iPhone OS 4.0 to developers today. As with all betas, the exact changes are unknown because of the NDA. However, Apple has already said that 4.0 (when it is available to the general public) will include multitasking, the ability to change the springboard background (what an idea!), and various new features, such as Apple’s GameCenter, and iAd, Apple’s response to the mobile ad market.
Like all betas there are bugs, and this release is not recommended for anyone but developers (who need to get their app to work on 4.0, and utilize the newer APIs and features). In the meantime, feel free to read through this thread to learn more about the bugs, and to post in this forum for more information about what is known of the 4.0 update.