Part of the announcement at today's media event was a demonstration of Apple's FaceTime video-conferencing features on OS X, allowing the many iOS devices out there that support FaceTime to connect with people running Macs.
FaceTime is similar to any other video-conferencing tool, and by Steve Jobs' own admission, there is not much to it: you receive or make a call, the person answers, and you both are able to see each other. The software will automatically show portrait or landscape view, depending on how the other person is holding an iOS device, but beyond that, there is not much else to the program.
If you want to try FaceTime, Apple has a beta version available for download on the FaceTime for Mac Web page. The installer just includes the FaceTime application, and there are no extensions, libraries, fonts, or other system resources included with the application. This means that if you want to uninstall it, all you have to do is remove the application from your Applications folder and optionally remove the preferences file for the application, which is called "com.apple.Facetime.plist" and is located in the /username/Library/Preferences/ folder.
The beta requires OS X 10.6.4 or later, which also means that it will work only on Intel-based Macs. While you do not need an iSight Webcam to run the program and view incoming video, you will need one to (obviously) send video. The program is a 13MB download and will take up 38MB of space when installed, and will not require a restart.
I suspect that sooner or later, the FaceTime and iChat videoconferencing features will be merged, but for now, they exist in these separate applications.