I'd like to know if Ubuntu will offer other Desktop environments as an option during installation. This is only should we decide that Unity is not for us.
Ubuntu (the stock CD) has never offered an option to install different desktops in the installer, you make that decision by selecting Ubuntu, or Kubuntu, or Xubuntu, etc.
Unity will be the default desktop if your hardware supports it. If your hardware does not support Unity, you will get the same desktop that you have today, GNOME 2.x., which is available as the "Ubuntu Classic Desktop" in the login screen. Users can also just log into the classic session if they prefer the older GNOME desktop.
For those asking if Ubuntu will ever have an option in the installer for people to choose whatever desktop they want then no, that will not be available in the default desktop cd. Of course you can use the alternate installer to preseed your install with the desktop you want or have a custom kickstart or whatever.
If someone were to make a community derivative with GNOME Shell (or one that does let the user choose) then you can find information on how to do that here.
No, this is not planned in any form.
The main reason is a distribution needs an identity and direction. Mark Shuttleworth has guided Ubuntu this far, and the direction he is setting going forward puts Unity firmly at the forefront. Offering alternate desktop environments or shells during install only adds to user confusion and weakens the identity of the project.
Gnome 2.x will remain as a fallback for hardware that does not support unity, much as it currently does on the Netbook remix.