What would a WWDC thread be without predictions?
1) Apple replaces the MacMini with a new product that will serve a similar purpose and market segment, much like the iPod Nano replaced the iPod Mini (no MacNano jokes please

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2) Leopard features an enhanced BootCamp that will allow for easier emulation of Windows programs within OS X. The requirement that the user purchase and install a copy of Windows will still apply.
3) Finder will no longer be the centre of OS X. It will be replaced by a version of Spotlight on steroids. The Finder will still be available, but it will not be running by default.
4) The iMac loses its chin and will allow users to swap hard drives as they do RAM. They won't be able to open up the back as they did with the very first iMac.
5) Leopard includes a graphical version of MacPorts that can also be used as a repository for open source .app programs.
6) Apple open sources its version of Java. Now that Sun's version is open sourced and Apple's Java 6.0 is over 6 month behind Sun's, Apple has to do this. Also, since Apple's devs are also working on the iPhone and Apple TV, they no longer have the resources to maintain a separate version of Java on their own. It will not open source the native Aqua code that's used by Swing, only the Java classes that access that native code.
7) Bonjour takes a prominent role in Leopard, as well as the AppleTV and the iPhone (this is inspired from roughlydrafted.com, so I can't take full credit for this prediction).
8) Now that all first party apps (Finder, Safari, iTunes) have had their themes unified, Apple will replace the Mac look n' feel with something radically different from Aqua, far more significant than the change to brushed metal from pinstripes and just as significant as the move to OS X itself.