Speculating on the Apple tablet
I think it's going to be based on the Apple Air, because of the battery design, with a big chunk of either SSD or generic Flash storage. With some new form of virtual keyboard layout. And a few extra touch moves.
The reason Java is not on the iPhone has little to do with Java and everything to do with Cocoa Touch being a fork without all the crufty Carbon stuff which still lies underneath Cocoa. And Flash was banned too because Flash 10 has privacy issues (advertising trackers) and uses way too much CPU. An iPhone with Flash would survive 12 hours, or something like that, if the user was a fan of Youtube. People would be mad as hell and blame Apple. Apple will probably use HTML5, Animated PNG/MNG/SVG and CSS3 as a replacement.
Basically "pure Cocoa" on the desktop is a myth created by people who only use the latest and greatest and who live inside Interface Builder. The reality is that a serious programmer has to support the operating system that existed before the current one. And that means in effect that people have now started to drop Tiger support. Just think about creating a Windows application that only supports Windows 7 or even Vista. OS X people are actually more forgiving than Windows users in this respect.
I think Apple will use Snow Leopard for the tablet though, unless the device is tiny and is a one hand thing. Then Cocoa Touch is more logical. Though a tablet would be a great showcase for the Snow Leopard resolution independence code.