Great post!
As a Mac user who also has SuSE Linux installed on his iMac, there are a number of things that i think OSX could add such as multiple window managers, desktop environments, already installed, a bazillion open source apps, etc. as it is you have to get Xwindows up and running and then do a lot of phutzing around with Fink or other things to make this happen.
Anyway, I'd welcome an OSX/Linux PDA from Apple not just an OSX one. That sharp will probably be my next machine. With linux's potential small footprint there should be no excuse to add that into the mix in an Apple PDA too(let the flames begin!). It just could be there as a sort of dual boot option for those who like to go both ways. With Linux making inroads throughout the rest of the world, as an inexpensive OS the rest of the world can afford, Apple might do well to keep the lines of communication open -- especially while arrogant Microsoft seems intent on trying to close as many doors that technologies like linux and Java open. From what i can gather Debian has the potential to become a base standard Linux -- I wonder what flavor is in that Sharp?
But when all is said and done I could understand if Apple wanted to avoid the PDA market until it could definitely demonstrate a better ability to generate profits not burn them. I would imagine that the iPod will in some way be a barometer of the potential marketability of such devices. Heck, give me an iPod that's also a cell-phone and a PDA and I am giving you the greenbacks tomorrow, because I don't want to carry round more than one device at a time -- already i feel like a camel just toting around my backpack, wallet, PDA, & cell-phone.