The technology behind My Avatar Editor is Adobe's Flash Player. This is used to render the vector-based character graphics and provide the interactivity to the interface that allows you to dynamically change and adjust those graphics.
One of the features used by the Mii Editor was the ability to save files created within the editor itself to the users hard drive. This feature required a certain version of the Flash Player (version 9) that was not supported on the Wii™. This prevented anyone from being able to use the editor on the Wii™ but the Wii™ already had the Mii™ Channel so it wasn't a big deal (I thought), and the advantages of being able to save an image from the editor to your own computer couldn't be passed up.
One of the limitations of this feature in Flash Player 9 was that it had a server dependency. In other words, to be able to save a file from a Flash Player application, you had to send data from the Flash Player to the server so the server could send it to the browser to invoke a download prompt for the user.
This was not only messy to deal with but also required server bandwidth to maintain. The same also applied to loading or opening files which was even worse since the server had to save the files before bringing them into the player.
The newest version of the Flash Player, currently in beta as of this writing, now supports an enhanced method of saving (and loading) files that bypasses the server altogether.
This greatly simplifies the file process on my end making it a much cleaner process. Additionally, the time to load and save files should be a lot faster. Best of all, you won't even need an active connection to the internet to make use of the load/save features since it's all handled client-side.
I'll be targeting Flash Player 10 for My Avatar Editor. Assuming it gets released before the final version of Flash Player 10, it will require using a beta version of the player to use.