Ubuntu 9.04 showed up in the Update Manager. It's as easy as clicking the "Upgrade" button to do an operating system upgrade. And of course my DSL connection has to die when I'm some 300 packages into the 1391-package download process. I had to power cycle the DSL modem, and it came back with a slower connection speed.
Browsing the release notes, the most obvious change I see is that the Ctrl-Alt-Backspace salute (to kill the X server) is now disabled by default. I don't think it's a good idea—I've had occasion to use it several times in Ubuntu 8.10. With the Ctrl-Alt-Backspace option out, my remaining options are: ssh in from another box (which would require me to turn around and slide my chair three feet), or the reset button (which will require me to bend down, press and hold the big button for several seconds). Neither is as convenient as the key combination.
On the other hand, I have never ever accidentally pressed the key combination in my fifteen years using Linux.