From a thread in GeoGebra User Forum:

You can export directly from GeoGebra 4.2 Beta by checking the “Export to HTML5 only” option.

At the moment it supports just one Graphics View. We are hoping to add Tools, Input Bar, Spreadsheet View etc in the future.

GeoGebraTube has experimental support for showing applets in GeoGebraWeb. On devices without Java, it will automatically use GeoGebraWeb or you can force it to by adding ?mobile=true on the end of the URL, eg

Please bear in mind that Tablets are slower than desktop computers, so if designing for them you should keep your construction as simple as possible.
(also we have not done any optimization yet)