This could be a fake, but a forum poster on the Xbox.com forums claims that the list below details the full extent of the Xbox 360 dashboard update. Thanks to Xbox 360 Rally

# Compatibility with 3rd party wireless controllers
# Compatibility with messenger pad
# Xbox Live Vision firmware upgrade, sharper image filtering
# Many more Vision Cam settings
# New screensaver settings - when away from Xbox 360, screensaver will play for optional time, before screen goes dark
# New Xbox Live Arcade blade - where you can download, play and learn about Xbox Live Arcade
# Brand new web based adver-games flash games which aren’t downloaded, but are playable bite sized games, accessed via new Xbox Live Arcade Blade
# New Screensaver option in Themes and Gamer Pics option in Marketplace blade
# New functionality with Messenger, works with chatpad
# New XNA Game Launcher for those that have the program, new options
# New language, time and date settings
# New advertising opportunities in gameplay
# Xvid support
# New favourites option via Xbox Live Arcade, can rate according to preference
# IPTV Functionality - support for TiVo like PVR options. Will be able to record TV direct. Options include source by channel, director, actor and distributor
# Filter friends list, will be able to filter friends via games
# New clan support - will be able to launch clan games direct from dashboard, can “fold” friends into new clan category in friends list
# Better translations in settings and other options for Japanese, German and Spanish languages
# New quick option to delete via X Button in memory viewer.
# Option for having downloadable In-game content not inform when completed download
# Option for having video content not inform when completed download
# Faster settings for viewing unfinished video content downloads
# New option in Games tab - beta and others, will be launcher for MMO and multiplayer betas
# New option in Marketplace Games tab, search by letter, can be inputted via text pad or console
# Smarter game settings - you can be messaged via publishers about new in-game content for your games
# Picture in picture functionality for DVD Drive and some other updated settings
# Improved USB Keyboard settings
# Improved voice chat settings in game and from dashboard
# Faster response times to Redeem Code settings
# Option to play a Redeemed Code game straight after redeeming code
# Console will now recognise low Hard Drive space, will inform you immediately if can’t download current item, will automatically download next available that is able.
# Increased space size for gamerpics and dashboard themes
# Themes can now be animated
# Themes may now have sound effects
# Better slideshow capabilities, new options added. New sound effects and integration with Hard Drive/ PC music capable.
# New estimated timer for downloads, informing how long download will expect to take

An official announcement and, we expect, confirmation that this list is correct is expected shortly. The update is apparently due on the 10th September.