I've taught a few kids using Scratch and it works quite well. I'm happy to hear they're working on an HTML5 version, and hopefully it will be possible to install if offline like the current Adobe AIR version does. I teach with a few netbooks running Ubuntu, and can't always rely on having internet, so I had to install the last version of AIR that supported Linux to get it working.

There is also an unaffiliated app for Android called Pocket Code that is clearly inspired by Scratch. It works nicely on Nexus 7 tablets, and touch screens are clearly more natural for kids, but it is buggy and more limited than Scratch so I had to stop using it. Hopefully it will improve, or the Scratch team will provide their own mobile port (not a simple task, of course).