I upgraded from Ubuntu 9.04 to 9.10 on my office desktop machine.  I 
started it in the office and continued remotely from home via ssh with X 
port forwarding and such (e.g., x0vnc4server).  In the end, it reboots one 
last time, so I don't think the way I completed the upgrade is relevant.

After it finished upgrading and rebooting, I used ssh to start Xvnc with 
icewm then I connected via vnc and started using it.

When I went to my office earlier today, the Gnome login screen had only 
the machine name in a rectangle in the middle and no login prompt.  I 
tried a few different keystrokes to make something happen, but nothing 
happened.  It seemed to be frozen.  I tried ctrl-alt-F1 and that let me 
log in to a console -- I can look at processes and such but how can I log 
into Gnome?  I know I can reboot but I prefer not to kill the working VNC 
session.  I don't know that rebooting will work, though.

I have to say that I also had some weirdness on a second machine where I 
upgraded to 9.10.  After reboot it used to know my username and it would 
prompt with it, but now lists "other..." several times instead of 
remembering me.  It seems that I have to choose the last "other..." for it 
to let me log in.  When I lock the screen it comes back with a big, bold 
"%R" in the top center of the box, which means what?  There seems to be 
something wrong there.

These might not be big problems but I'm not sure yet.  Otherwise Ubuntu 
has been going great for me.  The other day I had to reboot for the first 
time in 6 months (since the last upgrade), but it turned out that the 
problem was with Windows inside of VirtualBox taking my system down!  I 
should have known better than to keep any Windows around.

Mike