I booted up my old PowerPC running 10.4 the other day to update some software. I updated Firefox, Flash, Adium, etc. When I ran iTunes, it told me there was an update available for that, too.
It prompted me to download iTunes 10 - which isn't for Mac OS X 10.4. I was running an older 9.0 version, and I knew there were updates available, but iTunes only prompted me to install 10.0.
Supposedly, 10.4.11 was originally listed as a supported platform for iTunes 10.
iTunes 10 only works on Mac OS X 10.5 and newer.
The last version of iTunes that works on 10.4 is 9.2.1, and you can get it here: http://support.apple.com/kb/DL1056.
iTunes 9.x will continue to prompt you that 10.x is out, even if you are running on an unsupported platform.
To stop this, open Preferences and uncheck "Check for new software updates automatically".