Here is a nice tip provided by Frederic Serriere (Escalation Engineer, Citrix) on Doctor-Citrix.com :

To Log ICA Logons and Logoffs in the Server's EventLog you need to add the following registry value on the Server :

In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Citrix

Add a DWORD Value called wsxicaEvent with a value of 0x1

Note: EventID 2 for Logon and 3 for Logoff.