I had the same issue as Godhika's video demonstrates here - the thumb disappearing as soon as the mouse pointer gets near it.

Weirdly it seems to have resolved itself with the last batch of updates I applied just an hour or so ago.

So something in this lot from /var/log/apt/history.log resolved it (apologies for the formatting!)

Start-Date: 2012-03-20  13:55:25
Commandline: apt-get upgrade
Upgrade: libatk1.0-0:amd64 (2.3.93-1, 2.3.95-0ubuntu1), gir1.2-rb-3.0:amd64 (2.96-0ubuntu1, 2.96-0ubuntu3), libgtksourceview-3.0-0:amd64 (3.3.4-1ubuntu1, 3.3.5-0ubuntu1), console-setup:amd64 (1.70ubuntu3, 1.70ubuntu4), libcupsppdc1:amd64 (1.5.2-8, 1.5.2-8bzr1), python-pyatspi2:amd64 (2.3.5-0ubuntu2, 2.3.92-0ubuntu1), nautilus:amd64 (3.3.91-0ubuntu4, 3.3.92-0ubuntu1), libatspi2.0-0:amd64 (2.3.91-1, 2.3.92-0ubuntu1), libcupsimage2:amd64 (1.5.2-8, 1.5.2-8bzr1), rhythmbox-data:amd64 (2.96-0ubuntu1, 2.96-0ubuntu3), libcupscgi1:amd64 (1.5.2-8, 1.5.2-8bzr1), libglib2.0-data:amd64 (2.31.20-0ubuntu3, 2.31.22-0ubuntu1), at-spi2-core:amd64 (2.3.91-1, 2.3.92-0ubuntu1), libcupsdriver1:amd64 (1.5.2-8, 1.5.2-8bzr1), libatk1.0-data:amd64 (2.3.93-1, 2.3.95-0ubuntu1), glib-networking-common:amd64 (2.31.20-0ubuntu1, 2.31.22-1), fonts-opensymbol:amd64 (102.2+LibO3.5.0-2ubuntu1, 102.2+LibO3.5.1-1ubuntu1), libatk-adaptor:amd64 (2.3.91-1, 2.3.92-0ubuntu1), libcupsfilters1:amd64 (1.0.5-1, 1.0.6-0bzr1), rhythmbox:amd64 (2.96-0ubuntu1, 2.96-0ubuntu3), cups-client:amd64 (1.5.2-8, 1.5.2-8bzr1), ttf-opensymbol:amd64 (102.2+LibO3.5.0-2ubuntu1, 102.2+LibO3.5.1-1ubuntu1), librhythmbox-core5:amd64 (2.96-0ubuntu1, 2.96-0ubuntu3), gir1.2-gtk-3.0:amd64 (3.3.18-0ubuntu3, 3.3.18-0ubuntu4), libsoup-gnome2.4-1:amd64 (2.37.91-1, 2.37.92-1), libglib2.0-0:amd64 (2.31.20-0ubuntu3, 2.31.22-0ubuntu1), cups-ppdc:amd64 (1.5.2-8, 1.5.2-8bzr1), python-debtagshw:amd64 (1.9+git20120312, 1.9+git20120320), gir1.2-soup-2.4:amd64 (2.37.91-1, 2.37.92-1), gnome-icon-theme-symbolic:amd64 (3.3.91-1, 3.3.91.1-1), libgtk-3-bin:amd64 (3.3.18-0ubuntu3, 3.3.18-0ubuntu4), gnome-orca:amd64 (3.3.91-0ubuntu1, 3.3.92-0ubuntu1), cups-common:amd64 (1.5.2-8, 1.5.2-8bzr1), uno-libs3:amd64 (3.5.0-2ubuntu1, 3.5.1-1ubuntu1), libcups2:amd64 (1.5.2-8, 1.5.2-8bzr1), gir1.2-gtksource-3.0:amd64 (3.3.4-1ubuntu1, 3.3.5-0ubuntu1), libgtk-3-0:amd64 (3.3.18-0ubuntu3, 3.3.18-0ubuntu4), libgtksourceview-3.0-common:amd64 (3.3.4-1ubuntu1, 3.3.5-0ubuntu1), gir1.2-atspi-2.0:amd64 (2.3.91-1, 2.3.92-0ubuntu1), libglib2.0-bin:amd64 (2.31.20-0ubuntu3, 2.31.22-0ubuntu1), libgail-3-0:amd64 (3.3.18-0ubuntu3, 3.3.18-0ubuntu4), gir1.2-atk-1.0:amd64 (2.3.93-1, 2.3.95-0ubuntu1), cups:amd64 (1.5.2-8, 1.5.2-8bzr1), keyboard-configuration:amd64 (1.70ubuntu3, 1.70ubuntu4), nautilus-data:amd64 (3.3.91-0ubuntu4, 3.3.92-0ubuntu1), libreoffice-style-human:amd64 (3.5.0-2ubuntu1, 3.5.1-1ubuntu1), unity-greeter:amd64 (0.2.5-0ubuntu1, 0.2.5-0ubuntu2), cups-bsd:amd64 (1.5.2-8, 1.5.2-8bzr1), ure:amd64 (3.5.0-2ubuntu1, 3.5.1-1ubuntu1), libatk-adaptor-schemas:amd64 (2.3.91-1, 2.3.92-0ubuntu1), libreoffice-emailmerge:amd64 (3.5.0-2ubuntu1, 3.5.1-1ubuntu1), yelp-xsl:amd64 (3.3.3-0ubuntu1, 3.3.92-1), libcupsmime1:amd64 (1.5.2-8, 1.5.2-8bzr1), glib-networking-services:amd64 (2.31.20-0ubuntu1, 2.31.22-1), rhythmbox-plugin-cdrecorder:amd64 (2.96-0ubuntu1, 2.96-0ubuntu3), libnautilus-extension1a:amd64 (3.3.91-0ubuntu4, 3.3.92-0ubuntu1), cups-filters:amd64 (1.0.5-1, 1.0.6-0bzr1), libgtk-3-common:amd64 (3.3.18-0ubuntu3, 3.3.18-0ubuntu4), file-roller:amd64 (3.3.91-0ubuntu1, 3.3.92-0ubuntu1), glib-networking:amd64 (2.31.20-0ubuntu1, 2.31.22-1), libsoup2.4-1:amd64 (2.37.91-1, 2.37.92-1)
End-Date: 2012-03-20  13:58:25