iOS 4 is a free software update offering new features for iPhone 3G, iPhone 3GS, and iPhone 4 (which ships with iOS 4).
Features and capabilities vary between models, with some features available only on newer models.

Microsoft Exchange ActiveSync 14.1, included with Exchange Server 2010 SP1, introduces a default limit of ten devices per Exchange account. If you configure an iOS device that exceeds this per-account limit, you may see unexpected behavior:

Mail and Calendar accounts become unresponsive after creating a new Exchange account, or after the server for an existing Exchange account is updated to SP1.
When installing a profile that configures an Exchange account, the message "An unrecoverable error occurred while validating the account information" appears, and the profile fails to install.
When you check email on another device, you see the message "Error with your new mobile phone partnership," which says "You have 11 phone partnerships out of the maximum allowed 10 partnerships."

Before a new iPhone, Pad, or iPod touch can be used, it must be activated by connecting it to a computer that is running iTunes. During that connection, even if no other choices are made, the iTunes account on the computer can be synced over USB along with the information for activation.

You can use the iPad Dock Connector to VGA Adapter to connect your iPad, iPhone 4, or iPod touch (4th generation) to a compatible TV, projector, or VGA display.

In some cases, iPhone may not show up in iTunes under Devices while displaying the following error message: