Will there be a straight upgrade path from 10.10 to 11.04 (Gnome to Unity), and will there be measures put in place to ensure that Gnome is properly "cleaned" out on the way?
Unity is simply a new shell for Gnome and will probably be installed as part of the current ubuntu-desktop metapackage. This means that upon upgrading to Ubuntu 11.04 Unity will be installed by default with the upgrade.
I am assuming that Gnome 2 is planned to be installed as well, so users both upgrading and freshly installing will get both, just Unity by default.
We will of course provide an upgrade path, I don't know what you mean by "cleaned" so please provide more detail in your question.
In the past when we have major such changes (metacity to compiz, for example) to the defaults upgrades kept their current settings and new installs got the new stuff.
Having just upgraded from 10.10 to 11.04, it has replaced my 'old' desktop with Unity. Have had a loooong search to find out how to replace it (SystemSettings->System->LoginScreen - switch to 'Ubuntu Classic')
I believe that the statement was that "Unity will be the default UI for clean installs on hardware that will give a satisfactory performance" (or something like that), so that suggests that if you do an in-place upgrade, you will stick with stock GNOME