I'm currently researching ideas for the media management side of a CMS I'm writing. I was looking at having images served from a CDN which is fine on a single site, but I want all sites that run the CMS to make use of a CDN (which will most likely be a custom developed one, rather than a third party service like S3).

My main question is: Is a multi-site CDN a good idea? I can't think of a downside, but have probably missed something - obviously they won't share the same folder, as I invisage the requests to be css.cdnsite.com/example.com/style.css or something along those lines. Having multiple sites in the same place will obviously make it easier for us to manage, as well as being cheaper, but then I wonder if it'll be worth it...

Long story short: How should the CMS handle user uploaded media (separate installations)

Just keep a local copy of all assets and serve them from the same site, like in days of yore?

Keep a local copy, force site to use www. and have CDN subdomains per site?

Or use a single separate CDN for all sites?

Apologies for the length of this question, not sure if this should be multiple questions or not, as all parts are kind of related and could affect each other.