I've been running oohEmbed.com, a similar service, for more than two years now. The problem with doing this in client-side JS is that (a) the oEmbed spec doesn't specify json callbacks so oembed providers don't support callbacks making async stuff in the browser difficult; (b) dealing with SOAP APIs, html parsing, etc. are difficult to say the least within browsers.

But if you really want to do this without relying on an external service, you can take the oohEmbed.com source (BSD licensed) and run it as your own 'private' Google App Engine app. oohEmbed.com itself is App Engine hosted.