The iPhone 4, iPhone 3GS and iPod Touch 4th generation will be able to make and receive calls via Wi-Fi and 3G connections as long as you are running iOS 4.0, or above. The iPad and 3rd-gen iPod Touch will only be good for receiving video calls as they lack a camera. All devices will also be able to communicate with desktop clients running Windows, OS X or Linux.
There's no word yet on when a similar update for Android-based devices or other platforms will arrive, but we have to guess it's just a matter of time considering the increasing popularity of Google's mobile platform.
Despite some recent issues Skype experienced, the decentralized peer to peer nature of their network is known to be one of the most reliable and speedy for videoconferencing (not to mention free). This will no doubt pose a threat to Apple's highly touted Facetime feature that only works through Wi-Fi and between iPhones.
Surprisingly Apple has not enforced any restrictions on the revamped Skype app, nor have we heard any complaints from AT&T or other iPhone-exclusive mobile carriers (yet). According to Mac blog, 9to5Mac, a minute of a Skype video call uses ~3.4 MB of data per minute.