UPDATE: Screencast.com now has variable speed playback when viewing mp4 content with a fully HTML 5 Video complient browser.
One thing we are struggling with is that IE 9 & 10 are partially HTML 5 Video compliant. There are some core video features (full screen) that will not work when using HTML 5 and thus we will be adjusting Screencast.com to use Flash for video when viewed with IE 9 & 10. Upgrading to IE 11, or using Firefox, Chrome, or Safari, will allow HTML 5 to function fully. This means that in the near future, IE 9 & 10 will not have the variable speed playback due to lack of support. It is unfortunately the case that the older technology just doesn't have the robustness to support this new feature.
Other than that, watching content in high speed has been fantastic, I hope you enjoy it as much as we have!