Zend and Symphony Frameworks are actually the most adopted php frameworks in the world (and we have a Symfony appliance which is cool).
On the other hand, as far as i know, Yii is 3 or 4 in the list and is the php framework having the quickest gain of popularity. Maybe more important, it's a "simple" and fast framework. Upon these attributes, the proposition is to bring a Yii appliance :
Fast : to keep this quality and even enhance it, the idea is to package the most performant components with it : Nginx as the web server, PHP-FPM and APC. Regarding the free control panel supporting Nginx, i only know ISPConfig 3 http://www.ispconfig.org/, if you know better or have a better idea, that's very fine but ISPConfig has several interesting things like being light, bringing a remoting framework and managing multiple servers from one control panel.
Simple : it would be the real added value of a Yii appliance as having this configuration up and running can be tedious or intimidating and so would allow a huge community of developers to deploy in no time without headaches.