Currently I'm using the Jetpack plugin with mobile theme enabled for my WP site, which serves its basic purpose of converting your posts into a mobile-friendly format.
However, if you are looking at more customised mobile themes, you may want to look into responsive themes as phranque suggested. Such themes are designed to adapt your website to mobile devices naturally, keeping your design elements intact. Try looking at smashingmagazine.com!
Best of all, if you have the resources and expertise, create your own mobile app, like what mashable.com did. This is probably the best way to customise your website for mobile users for now.
But given that you have 100+ WP sites, you might want to use a plugin to make them mobile friendly first (but at the cost of losing your design elements), and start looking into responsive themes for each of them.