If I make an Amazon s3 MP4 resource publically availible and then throw the Html5 Video tag around the resource's URL will it stream? Is it really that simple. There are a lot of "encoding" api's out there such as pandastream and zencoder and I'm not sure exactly what these companies do. Do they just manage bandwidth allocation(upgrading/downgrading stream quality and delivery rate/cross-platform optimization?) Or do encoding services do more then that.
This is Brandon from Zencoder. What you're looking for is probably something like Video JS (videojs.com) for video playback. You can just upload an MP4 to S3 and reference it in a player (or the video tag directly, but that has additional issues). Our service is actually used for transcoding the video itself, not delivery. We actually created Video JS to help our customers (and the web at large) with easy, compatible HTML5 playback. If you have any other questions just ask. Thanks.
Answer to first part of your question is, YES it is really that simple. There is a howto about it and a working demo at the end of the article that you can see as a proof of concept.
Hope this helps.
Amazon S3 is a really good choice for serving up video content. We've been using it for a couple of years with no issues and the cost has been unbeatable. You should also look at using Amazon CloudFront and configuring your media to use their "streaming distributions". It basically uses your S3 files but copies them to edge locations around the internet and uses RTMP to provide a better playback experience for users and to save you money on bandwidth.