I recently encountered some troubles when upgrading to version 9.04 of Kubuntu (Jaunty Jackalope). 3/5 machines that I attempted to upgrade went smooth for the most part, only having troubles with downloading the packages because of constant timeouts and failures which I attribute to "launch day" overload issues. The two other machines I have upgraded so far were a different issue.
Laptop (Dell Lattitude 131L)
The first machine issue I had was a Dell laptop. After the upgrade and reboot had completed, I was left with an non-functioning x-installation. X would appear to come up, but would display a completely unusable set of random colors on the screen. After attempting several times to simply reset the X-settings, I needed to try something new. I took the time to uninstall all of the dbus and xserver packages (sudo apt-get purge dbus; sudo apt-get remove xserver*), and then re-installed kubuntu-desktop with sudo apt-get install kubuntu-desktop --reinstall. Althought this didn't appear to work, and a few hours went by as I tried other things, I ended up running those commands again, and then reconfiguring X with sudo dpkg-reconfigure xserver-xorg -phigh and everything began working again.
My Main Machine (Custom Built AMD 5200+ with 6GB of RAM)
The other machine that had issues was my primary machine, the one with the most power, my RAID array, and all of my media. This machine had problems with X as well, but on this machine whenever X would attempt to come up, the entire computer would freeze, probably due to some type of kernel panic. In this case I wasn't even able to use Sys_Req key to return to the console, and I had to hard-power off the computer. After running all of the same commands as with the first computer, things still weren't fixed. I eventually discovered that I could run the computer just fine by entering recovery mode, then dropping to a shell, then manually starting dbus and hal, and then starting KDM. This solution was obviously not ideal, so I continued looking.
Attemting to boot into a live CD was very revealing. I determined that I experienced the same problem with the 9.04 X64 desktop live CD. On a whim, I tried the 8.10 live CD which I knew had worked in the past, but this experienced the same issue. This was alarming, but I eventually determined that I could run both of the live CDs with the kernel flag acpi=off. I attempted to add this to my menu.lst file, and everything began to work again.
Although I'm not sure why 8.04 and 8.10 worked without this flag, I have noticed that my machine works more quickly and more smoothly than ever before. I think it is possible that some sort of timeout or error was continually ocurring under my previous kernel installations, and that now that my computer isn't attempting to run acpi, things just work. The places I notice the difference the most is in graphical effects from KDE 4.2. Windows are created and fade away smoothly, my windows wobble and resize better than before, and there is no jittering that I thought was just a normal part of graphics acceleration.
On a final note, even my keyboard settings have been fixed. Previously when I went into KDE's system settings->Keyboard options, I was unable to specify the state I wanted NumLock in upon startup, but now the option is there and it is working great.