cron/dbus problem?

[Dennis Jacobfeuerborn]

Given the (puzzling) absence of an alarm/reminder applet for the gnome 
panel I tried (ab)using Tomboy for this purpose but hit a problem involving 
Cron and DBus. When I call "tomboy --open-note Alarm" the respective notes 
pops up but when I put the same command in the crontab I get this:

Unhandled Exception: DBus.DBusException: Unable to determine the address of 
the message bus
in <0x0009f> DBus.Bus:GetBus (BusType busType)
in <0x00009> DBus.Bus:GetSessionBus ()
in <0x00022> Tomboy.TomboyCommandLine:Execute ()
in <0x00061> Tomboy.Tomboy:Main (System.String[] args)

Does anyone have an idea why the session bus seems to be available from a 
regular shell but not from cron?

Regards,
   Dennis