From Apple.com:

In the box

• Apple TV
• Apple Remote
• Power cord
• Quick Start guide

Size and weight

• Footprint: 7.7 by 7.7 inches (197 by 197 mm)
• Height: 1.1 inches (28 mm)
• Weight: 2.4 pounds (1.09 kg)1

Power

• Built-in 48-watt universal power supply

1. Actual weight varies by configuration and manufacturing process.
2. 1GB = 1 billion bytes; actual formatted capacity less.
3. Video capacity based on H.264 1.5-Mbps video at 640-by-480-pixel resolution combined with 128-Kbps audio. Actual capacity varies by content.
4. Music capacity based on 4 minutes per song and 128-Kbps AAC encoding. Actual capacity varies by content.
5. Photo capacity based on Apple TV-viewable photos transferred from iTunes.
6. Based on an IEEE 802.11n draft specification. Learn more about the 802.11n draft specification.
7. 
   

Internet access is required and a broadband connection is recommended.

Other Details:

What we know so far:
"Hardware:

• 1.0GHz Intel Pentium M-based "Crofton" CPU, based on SSE2 Dothan model # 7645A966 0159
• NVIDIA G72M graphics with 64MB DDR2 memory
• 256MB of 400MHz DDR2

Software:

• Modified version of OS X 10.4.7 along with some 10.4.9 kexts

Firmware:

• Apple TV uses EFI, based on the presence of the hard drive in GUID map.

Communication:

• TCP port 3689 is used to communicate with iTunes while using the iTunes Library Sharing feature.
• UDP port 5353 is used by Apple TV for automatically finding computers with iTunes on your network using Bonjour.
• TCP port 80 and 443 are used for basic and secure communications with the iTunes Store via the Internet.

Formats:

• H.264 and protected H.264 (from iTunes Store): Up to 5 Mbps, Progressive Main Profile (CAVLC) with AAC-LC audio up to 160 Kbps (maximum resolution: 1280 by 720 pixels at 24 fps, 960 by 540 pixels at 30 fps)
• iTunes Store purchased video: 320 by 240 pixels or 640 by 480 pixels
• MPEG-4: Up to 3 Mbps, Simple Profile with AAC-LC audio up to 160 Kbps (maximum resolution: 720 by 432 pixels at 30 fps)"