There would almost certainly be some sort of discount on the desktop version for PDA-version users, yes; it just makes intuitive sense, really, the product is less valuable to someone who already owns Pleco on a handheld than it is to someone who doesn't own any version of our software. Integrated web browsing is one of the key features we think this would add over the PDA version, and we've got some very cool ideas of things we can do with vocabulary on web pages. And of course management of / export to your PDA flashcards would be a key feature too.
A Mac version is a definite possibility, though between the ease of running Linux apps in X11 on Mac, the growing popularity of Linux on desktops, and Apple's generally-horrible attitude towards developers as so recently demonstrated with the iPhone firmware fiasco, I'm starting to think that we might be better off supporting Linux first. Ultimately it's a business decision, though, and since Mac remains popular in schools and it's unlikely Apple's going to make OS X development web-only anytime soon it probably does make sense to do a dedicated Mac version assuming the Windows one is at all successful, however personally annoyed I may be with the way they're handling the iPhone.
Pronunciation testing would be tricky, we'd probably have to license a lot of the technology since it's beyond what we can develop ourselves, though certainly something basic which tested you on your tones by measuring pitch changes might be within our abilities. But yes, in general we'd be trying to add a lot of features that aren't possible on handhelds. A free version for flashcard management also makes sense, though since we probably wouldn't be able to include any licensed content in that free version you'd be limited to either finding words in a free dictionary like Adso/CEDICT or entering them manually.