I have the same problem after upgrading to Maverick (only a thin line appears on the Gnome Panel and I can't run the Time Tracker app from the menu). I ran "/usr/lib/hamster-applet/hamster-applet -w" and got the following (version info is below):

** (hamster-applet:7315): WARNING **: Trying to register gtype 'WnckWindowState' as enum when in fact it is of type 'GFlags'

** (hamster-applet:7315): WARNING **: Trying to register gtype 'WnckWindowActions' as enum when in fact it is of type 'GFlags'

** (hamster-applet:7315): WARNING **: Trying to register gtype 'WnckWindowMoveResizeMask' as enum when in fact it is of type 'GFlags'
2010-11-16 16:26:22,494 ERROR: Introspect error on :1.217:/org/gnome/Hamster: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.NoReply: Message did not receive a reply (timeout by message bus)
2010-11-16 16:26:22,501 ERROR: Traceback (most recent call last):
  File "/usr/lib/hamster-applet/hamster-applet", line 120, in <module>
    applet_factory(applet, None)
  File "/usr/lib/hamster-applet/hamster-applet", line 35, in applet_factory
    hamster_applet = HamsterApplet(applet)
  File "/usr/lib/pymodules/python2.6/hamster/applet.py", line 203, in __init__
    self.new_name = widgets.ActivityEntry()
  File "/usr/lib/pymodules/python2.6/hamster/widgets/activityentry.py", line 102, in __init__
    self.populate_suggestions()
  File "/usr/lib/pymodules/python2.6/hamster/widgets/activityentry.py", line 225, in populate_suggestions
    self.activities = runtime.storage.get_activities(input_activity.activity_name)
  File "/usr/lib/pymodules/python2.6/hamster/client.py", line 135, in get_activities
    return self._to_dict(('name', 'category'), self.conn.GetActivities(search))
  File "/usr/lib/pymodules/python2.6/dbus/proxies.py", line 68, in __call__
    return self._proxy_method(*args, **keywords)
  File "/usr/lib/pymodules/python2.6/dbus/proxies.py", line 140, in __call__
    **keywords)
  File "/usr/lib/pymodules/python2.6/dbus/connection.py", line 620, in call_blocking
    message, timeout)
DBusException: org.freedesktop.DBus.Error.ServiceUnknown: The name :1.217 was not provided by any .service files

Version info: sudo apt-cache show hamster-applet:

Package: hamster-applet
Priority: optional
Section: universe/gnome
Installed-Size: 2988
Maintainer: Ubuntu Desktop Team <email address hidden>
Original-Maintainer: Pedro Fragoso <email address hidden>
Architecture: i386
Version: 2.31.92-0ubuntu2
Depends: gconf2 (>= 2.28.1-2), python (>= 2.5), python-support (>= 0.90.0), python-cairo, python-gtk2 (>= 2.12.0), python-gconf, python-gnomeapplet (>= 2.10.0), python-evolution (>= 2.10.0), python-dbus, python-gobject (>= 2.14.0), python-gnome2, python-wnck
Recommends: python-notify
Filename: pool/universe/h/hamster-applet/hamster-applet_2.31.92-0ubuntu2_i386.deb
Size: 418546
MD5sum: d5cd72d36449cc404f37d617356b17cb
SHA1: e2b93a8ee98426fe8caf58c0632a82ecc5232470
SHA256: ed622d054504086f2427c9e544e45387356d03ef5db1cb8f562e208c0bc64a02
Description: time tracking applet for GNOME
 Project Hamster helps you to keep track of how much time you spend on various
 activities during the day.  Whenever you move from one task to another, you
 change your current activity in the GNOME applet.
 .
 It can present graphical statistics of how long you have spent on each task,
 and may be useful for project management or keeping employee timesheets.
Homepage: http://projecthamster.wordpress.com
Python-Version: 2.6
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Origin: Ubuntu