I've been playing around quite a bit with the new Vine app, which lets you post six-second looping videos to your Twitter stream or other social media service. You can create animations or employ other goofy effects, but everything must be shot in order. No after-the-fact editing is possible.
Something else that doesn't seem to be possible, as many disgruntled users are discovering, is reuploading a Vine that fails to upload in the first place. If your upload fails, it looks like you're shit out of luck. I found this out on Saturday morning when a Vine I'd been planning in my head for days failed to upload. If I could have taken the Vine app out of my iPhone and smashed the code on the sidewalk, that's just what I would have done.
Rather than trying to reshoot my video, though, I found a workaround. Vine does save your little square video to your phone, and from there it can of course be uploaded to other video-sharing services. YouTube doesn't seem to allow embedded videos to loop, but Vimeo does, so that's where my lost Vine now lives: