iOS Question:

If you’re looking to update to iOS 7 when it lands in the fall, you’ll need to have one of these devices: the iPhone 4 or later; iPad 2 or later; iPad mini; or fifth-generation iPod touch. Oh, and both the iPhone 5s and iPhone 5c announced by Apple will arrive with iOS 7 already installed, though they don’t go on sale until September 20—two days after iOS 7’s release.

Specific features have more stringent device requirements (see below for details on these features):

Panoramic photos are only available on the iPhone 4S or later and fifth-generation iPod touch.
You can’t shoot square photos or video using the iPad 2; all other devices can use this feature, however.
For the nifty new live camera filters, you’ll need an iPhone 5 (including the s and c models) or fifth-generation iPod touch. Putting filters on after the fact in the Photos app is an option for the iPhone 4 or later, third-generation iPad or later, iPad mini, and fifth-generation iPod touch.
To AirDrop a photo or other file to your friends, you’ll need an iPhone 5 or later, fourth-generation iPad or later, iPad mini, or fifth-generation iPod touch. You’ll also need an iCloud account.
Siri continues to be limited to those using an iPhone 4S or later; third-generation iPad or later; iPad mini; and fifth-generation iPod touch. It also may not be available for your country. And if it’s a new Siri voice you desire, keep in mind that it’ll only be available initially in English, French, or German.