I was wondering since Ubuntu 11.04 will include Unity as the default interface, if (when the time comes) I upgrade to 11.04 from 10.10 will my current desktop setup be preserved somewhere, so in case I do not like Unity I can simply switch back to my Gnome desktop
Switching between Unity and your (preserved) GNOME session will be as simple as logging out, choosing "Ubuntu Classic Desktop" at the login screen, and logging in again.
Note that session saving is not present in 11.04: see https://bugs.launchpad.net/ubuntu/+source/gnome-session/+bug/726431 . I'm not sure if a session you saved in 10.10 before upgrade would be restored, if that's what you mean.