IBM is doing a great job, but is it a good idea to rely upon a single supplier? Also, while IBM and Apple are friendly right now, who knows what the future holds? IBM might get annoyed if Apple performs a miracle and gains market share for high end servers, for example. Also, relying on a single chip fab means being at the mercy of the production limits for that fab, as well as sharing with other customers.
Maybe Apple should ask IBM to help build their own fab. The investment required would be huge but a shared use fab with IBM might work. Plus, the new fab could include all of the cutting edge tech such as a .065 micron process, silicon on insulator chips, double gates, etc. A PowerPC 980 chip could easily run at 10 Ghz with production in early 2007. IBM has already been testing chips similar to these with very promising results.
Sun might also want to partner, especially if the fab is built in Texas near their production HQ at TI. Either build new Sparc chips at the fab or convince Sun to switch to PowerPC.
For that matter, perhaps Apple should acquire Sun via a leveraged buyout. Using this method it is possible for a company to acquire another company Solaris shares it's BSD origins with OSX so providing support for legacy Solaris Apps could be possible. More recent SYSV based apps could continue to be supported on existing hardware while newer apps could be built for newer hardware. This could turn into a major migration, as in the move from OS9 to OSX. This would be a dramatic change for Sun, just as it was with Apple. The potential benefits might be worth it for Apple as the acquirer, if not for Sun. Apple would gain a large pool of talented employees and greatly expand their market share.
(edited for clarity- sorry about the confusion)