Cocoa + Quicktime provide very powerful APIs for video/audio processing. I don't know of any legally clean way to say, encode AAC files and provide them to you

Cocoa + Quicktime provide very powerful APIs for video/audio processing. I don't know of any legally clean way to say, encode AAC files and provide them to your users without paying a royalty rate. You could just have the server run a script that passes it through Quicktime Pro.