In 11.10, it should automatically start in 2D mode if 3D acceleration isn't available. Alternately, you can choose it from the login screen. Here is how Unity and Unity 2D are configured on my system.

    $ cat /usr/share/gnome-session/sessions/ubuntu.session 
    [GNOME Session]
    Name=Ubuntu
    Name[eo]=Ubuntuo
    RequiredComponents=gnome-settings-daemon;
    RequiredProviders=windowmanager;panel;
    DefaultProvider-windowmanager=compiz
    DefaultProvider-panel=compiz
    IsRunnableHelper=/usr/lib/nux/unity_support_test
    FallbackSession=ubuntu-2d
    DesktopName=Unity

    $ cat /usr/share/gnome-session/sessions/ubuntu-2d.session 
    [GNOME Session]
    Name=Ubuntu 2D
    RequiredComponents=gnome-settings-daemon;
    RequiredProviders=windowmanager;panel;launcher;
    DefaultProvider-windowmanager=metacity
    DefaultProvider-panel=unity-2d-panel
    DefaultProvider-launcher=unity-2d-launcher
    DesktopName=Unity

Ok, in 11.04, without acceleration, I have to switch back to "classical" mode which I assume to be Gnome.

> sudo apt-get install unity-2d

You may have to add this PPA first: http://www.ubuntuupdates.org/ppa/unity_2d?dist=oneiric

Then choose Unity 2D in the window manager options from the login manager.

This isn't necessary in Oneiric. Unity 2D is included and is the default fallback if 3D isn't supported.