I have a requirement to support multiple CDNs for different asset types (due to cost/volume consumption/politics) - I am assuming (as I cannot see any documentation) that SDL Media Manager will allow me to easily define a separate CDN target for the different MIME types?
As far as I understand it, Media Manager is only provided as a SAAS solution, so the assets are not hosted on your server, and you do not have any control over how they are provisioned (via CDN etc.). Therefore I dont think you have control over what CDN to use, unless you want to build something like Lars says to pull the assets from Media manager, and publish them to a CDN. That seems to destroy half the point of the SAAS offering though...
This is probably the wrong place to ask about core SDL Media Manager features.
Assuming you are referring to a Tridion integration with SDL Media Manager Connector (since this is the Tridion stackexchange site) and if SDL Media Manager can't do what you need it would technically be possible to do the following:
Write a custom TBB that uses the Media Manager Connector API (for 2011SP1, for 2013 this would be the ECL API - same thing, new name) to retrieve a link to the published images or files. Then copy them to your CDN of choice and insert the updated link in the template. I would not try to do anything with videos - the link you can retrieve is not to the video file directly, but to a player that access the video.
The main drawback is obviously that you now have to (re)publish the item through Tridion before it is available.
Notice as mentioned this is technically possible. Whether you should do it is another issue altogether which I won't make any comments on (although I did make this comment which might be seen as a hint). :)
SDL Media Manager comes with a CDN as it is a hosted service/SAAS. The CDN is included in the price and there is no need to bring your own CDN. If you want to have your data one particular CDN, I think Media Manager does support other CDNs on request by a CDN connector. Separating CDN targets for different MIME types is not supported.
Media Manager is a MAM, it focusses on the delivery of video and image assets. It's much more than a CDN, I think it cannot be replaced with a CDN + ECL connector. You would miss a lot of features, like localization with subtitles and audio tracks, screen grabbing, multi chapter video's, the encoding for various formats and devices, the video player and so on. Only if you need a CDN to host some static images, then indeed you can use a CDN and ECL, but you probably choose Media Manager for a reason.