anaconda by default now refers to encrypted devices by UUID, not by device name, to better handle device name changes. An encrypted device using partition /dev/sda1 with UUID 12345678-1234-1234-1234-123456789012 would now be called /dev/mapper/luks-12345678-1234-1234-1234-123456789012 instead of /dev/mapper/luks-sda1. You may wish to make the same changes on your system on upgrade; if you do, make sure to run mkinitrd afterwards.
If you use non-root encrypted devices, you will need to re-run mkinitrd for any older kernels that you want to still boot on the system.
It's been reported that there are problems with Pidgin i.e.:
pidgin-2.5.2-2.fc9.i386 from installed has depsolving problems - Error: Missing Dependency: libedataserver-1.2.so.9 is needed by package pidgin-2.5.2-2.fc9.i386 (installed)
If you are running as a xen Host (Dom0) you should not upgrade to Fedora 9. 
Upgrading the thunderbird package will fail . The current workaround is yum remove thunderbird; yum install thunderbird>.
If you are upgrading to Fedora 9 and use emacs, you must upgrade to the latest version of emacs for your prior release to ensure a clean upgrade. Fedora 8 users must have emacs-22.1-10.fc8 or later, while Fedora 7 users must have emacs-22.1-7.fc7. Bug report here .
Due to the switch from sysvinit to Upstart , it is recommended that users who do an upgrade on a live filesystem to Fedora 9 reboot soon afterwards. If you do not reboot, you may want to take careful note of any '/etc/inittab.rpmsave' or '/etc/inittab.rpmnew' files. When rebooting after an upgrade then be aware that sysvinit that was used to start the system can't be used to shut it down.