I did the Apple TV ATV Flash hack yesterday, and love it. It still streams beautifully. Now, I'm just transferring my non MPEG-4 movies to the Apple TV HD via FTP, it's only taking me about 10 minutes for a compressed 700 MB movie. You can also stream them or use an external HD if you want. This program adds a ton of features. The best thing is, it doesn't change the interface, it just adds to it. While making the USB file, you have to check the box that says "Do not autoupdate", or you run the risk of having to install it again as the Apple TV's automatically check for updates weekly (although I don't think they update on their own).
http://www.atvflash.com/