Same here. After upgrading from 11.04 to 11.10 to 12.04 (beta2) the DASH menu doesn't list any applications. Previously typing "calc" for example would list the calculator and LibreOffice Calc. Now, nothing shows up, neither in the main DASH window, nor in the application lens.
However, in the main lens songs (FLAC) do show up, but for instance documents don't. Furthermore, I have regular crashes of unity-lens-video. Apport told me this problem had already been reported. Maybe this is related?
Looking a bit closer in ~/.xsession-errors I found that the zeitgeist daemon crashed:
(zeitgeist-datahub:2329): LibZeitgeist-CRITICAL **: Unable to connect to Zeitgeist daemon: Error calling StartServiceByName for org.gnome.zeitgeist.Engine: GDBus.Error:org.freedesktop.DBus.Error.Spawn.ChildExited: Process /usr/bin/zeitgeist-daemon exited with status 1
According to https://bugs.launchpad.net/ubuntu/+source/zeitgeist/+bug/986191
this happens when upgrading from Ubuntu 11.04 to 12.04. Removing
~/.local/share/zeitgeist/activity.sqlite solves the problem. I tried it and it works :-).