In the version of i3 that came with Ubuntu 10.10, there was a bug that caused it to crash when running with Gnome. See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=610143 . However, in the version of i3 that comes with Ubuntu 11.04, the bug is fixed.
The way I got i3 to run as the WM for gnome (i.e. i3 + gnome-panel) was to go run gconf-editor, navigate to /desktop/gnome/session/required_components, and add/edit the key called windowmanager to have the value i3 (it was previously metacity or compiz). Next time you run Gnome (now "Ubuntu Classic") from GDM, it will use i3 as the window manager. For some reason, though, it takes like 30 seconds to start up the panel, so be patient.
When I upgraded to Ubuntu 11.10 and 12.04 I had to use a different solution (which works for both). I created a file called
/usr/share/gnome-session/sessions/gnome-i3.session with the followingcontents:
Then at the lightdm login screen, I just select gnome-i3 instead of "Ubuntu 2D" or whatever else is in the list. I may have had to install gnome classic/fallback as well, but there should be plenty of tutorials online on how to get that installed if it's not there already.
Another thing to note: when I upgraded i3 to 4.2 (I use the i3 Ubuntu repository), whenever I logged in the gnome-panel wouldn't appear until after I launched my first application.