If you're upgrading to 10.04 Lucid, KDE4 will crash at boot. To recover your system, just do the following steps:
- Press Alt-F2, type "konsole"
- At prompt, type "sudo rm /usr/lib/kde4/plasma_wallpaper_wallyplugin.so"
- Restart your system.
- KDE4 should start normally. Please remove Wally.
I'm preparing Wally packages for Lucid, they should be available ASAP.