Now, it seems a full list of Xbox 360 specs and Xbox Live details has been leaked. This list comes from a thread on the Team Xbox forums:

Xbox Live, 2 types: 1) Xbox Live Silver (no subscription required) 2) Xbox Live Gold (subscription benefits)

Features for Gold service: * Also for Silver // # Also for Offline

- Seamless transition to Xbox Live account from Xbox to Xbox 360
- Access to MMOs (additional fees may apply) *
- Free Xbox Live weekends *
- Multiplayer online gameplay
- Avatar for gamer profile * #
- Motto for gamer profile * #
- Personalized look for Xbox System Guide * #
- Offline achievments * #
- Online achievements *
- Access to other players' Gamer cards via Live *
- Cumulative gamer score * #
- Location/language profile * #
- Reputation *
- Enahnced matchmaking using above
- Skill level matchmaking
- Gameplay style profile (casual, competitive, etc.)
- Recent players list *
- Free and premium download game content *
- Free and premium downloadable movies, music, tv *
- Downloadable demos/trailers *
- Microtransactions *
- Custom playlist in every game * #
- Play music from portable devices * #
- View images from digital camera * #
- Strem media from Windows XP * #
- Interactive screen savers * #
- Track info for CDs * #
- Communication with voice, video or text *
Now for the hardware specs a.k.a. numbers:

1. Support for DVD-video, DVD-Rom, DVD-R/RW, CD-DA, CD-Rom, CD-R, CD-RW, WMA CD, MP3 cd, JPEG photo CD
2. All games supported at 16:9, 720p and 1080i, anti-aliasing
3. Customizable face plates to change appearance
4. 3 USB 2.0 ports
5. Support for 4 wireless controllers
6. Detachable 20GB drive
7. Wi-Fi ready

Custom IBM PowerPC-based CPU
- 3 symmetrical cores at 3.2 GHz each
- 2 hardware threads per core
- 1 VMX-128 vector unit per core
- 1 MB L2 cache

Custom ATI Graphics Processor
- 500 MHz
- 10 MB embedded DRAM
- 48-way parallel floating-point shader pipelines
- unified shader architecture

Memory
- 512 MB GDDR3 RAM
- 700 MHz DDR

Memory Bandwidth
- 22.4 GB/s memory interface bus bandwidth
- 256 GB/s memory bandwidth to EDRAM
- 21.6 GB/s frontside bus

Audio
- Mulitchannel surround sond output
- Supports 48khz 16-bit audio
- 320 independent decompression channels
- 32 bit processing
- 256+ audio channels