UPDATE January 20, 2004: Installing iTunes 4.2 from the iLife '04 CD-ROM, which you must purchase, seems to work fine, and all my workarounds below are unnecessary. It's only the free download that has trouble.
A warning for users of older Macs: if yours has a processor slower than 400 MHz, the new iTunes 4.2 may refuse to install on it, even though it can run just fine, as iTunes 4.1 does. Now that the latest versions of the Mac OS won't install on these old Macs (Mac OS X 10.3 "Panther" won't install on any beige Power Mac G3), Apple seems to be phasing out support from other software as well.
I have been using iTunes (and before that, its predecessor SoundJam) since it was first developed. Every version up to iTunes 4.1 has installed and run just fine on my 1997 Revision A 266 MHz Power Macintosh G3, currently running Mac OS X 10.2.8. I downloaded iTunes 4.2 (a genuinely minor update) when it was announced at Macworld, but the installer says that it "cannot be installed on this computer" and refuses to go any further.
I understand that the program is not officially supported on machines slower than 400 MHz, according to the system requirements. However, every version previous to this one has installed and run quite acceptably, playing and encoding MP3s and AAC files, playing CDs, displaying artwork, browsing the iTunes Music Store, and so on.
Most importantly, with a third-party utility (Pacifist) I was able to extract the files from the .mpkg download and install iTunes 4.2 on my Power Mac. It works just as well as previous versions. Even if it is not supported, therefore, it does work, and I would appreciate it if the installer did not prevent me from installing it on a system (Mac OS X 10.2.8) on which it will run.
I hope there are not similar problems with iPhoto 4 (which is supposed to run on 10.2.8) in the iLife 4 package—previous versions have also been unsupported on my sort of machine, but they do install and run without fuss. [UPDATE January 20, 2004: No problems there. iTunes 4.2, iPhoto 4, and iMovie 4 install from CD-ROM on my beige G3 just fine.] For people who can't afford a new Mac yet, if Apple arbitrarily decides not to let programs install or run when they will work functionally, the company is looking at losing some sales: songs we won't buy from the iTunes Music Store, iPods we won't purchase, iLife packages that won't be ordered. And I'm sure Apple would rather make some sales than not.
Then again, it could be a bug, which would be nice in a way.