It sounds like you're applying post-decode linear transformation to the data, but, if so, the video would only appear stabilized in your own custom player. Does MPEG-4 allow one to specify per-frame linear transformations beyond simple ROI? I didn't think so, but if so, that's pretty cool. Or perhaps you keep the original data around server-side, but always stream a corrected version to anyone who asks to play the video.
Either way, your videos look great. I love the fact that you also compensate for rotation, which is expensive even when you have macroblock vectors. I hope you can stay ahead of Apple (both iPhone, iMovie, and FCP) and Android in this space. Best of luck. May you become the Instagram of online video!