http://www.idpf.org/epub/20/spec/OPS...htm#Section3.1
Reading Systems must support all CSS2 selectors, including pseudo-elements and pseudo-classes. However, certain pseudo-classes may not be applicable to all Reading Systems (e.g. hover for devices with a touch screen) and may not result in any rendering on such devices.
And :first-letter is indeed a CSS2 selector:
http://www.w3.org/TR/2008/REC-CSS2-2.../selector.html