Right now, we don't have plans for the PDF version... but that's an interesting idea, and we'll bring it up with the publisher. It won't be on Safari because Safari is for text-based books, that can be rendered in XML. That's impossible to do with a Head First book, because they are graphics-based, not text-based. Think of a Head First book as 700+ pages of individual hand-outs, rather than a flow of continuous text. There IS no continuous text flow. It's a *designed* book rather than a *written* book... that's the best way I know how to describe the physical aspects of it. So unfortunately, there isn't yet a way to handle that in Safari.
Co-author of SCMAD Exam Guide, Author of JMADPlus SCJP1.2, CCNA, SCWCD1.4, SCBCD1.3, SCMAD1.0, SCJA1.0, SCJP6.0