Minimum requirements for mirroring Mac/PC in Apple TV
You can mirror your Mac's screen, Apple calls it AirPlay Mirroring. The section on the Apple TV page you mention actually refers to streaming your "photos, home videos, and music collection", which you can do via iTunes on a Mac or PC.

Requirements

Per Apple's support article on AirPlay Mirroring, to use it you need:


An Apple TV (2nd or 3rd generation) with software update v5.0 or later
And one of these Macs, running OS X 10.8 (Mountain Lion):

iMac (Mid 2011 or newer)
Mac mini (Mid 2011 or newer) 
MacBook Air (Mid 2011 or newer)
MacBook Pro (Early 2011 or newer)



Apple doesn't provide a means for Windows PCs to do AirPlay mirroring, but you might be able to find a third-party solution.

Networking

The Apple TV and the Mac need to be on the same network, one will not be automatically created, but you should be able to create a Wi-Fi network from your Mac and have the Apple TV join it if you don't have a network already set up.

Quality &amp; Performance

The quality is not the same as if you just plugged your Mac in via HDMI, it transcodes your desktop to a video format the Apple TV can read (H.264), since sending uncompressed video would require more bandwidth than any home network could provide. Quality and performance can depend on your Mac and network speed. I haven't tested it thoroughly, but I suspect it wouldn't be appropriate for gaming, as there would be latency issues.