You mentioned Win32::OLE but also mention "WMI" in your title. If you're using an external program, how about 'query':

VinsWorldcom@C:\Users\VinsWorldcom> which query
C:\Windows\system32\query.exe

VinsWorldcom@C:\Users\VinsWorldcom> query /?
QUERY { PROCESS | SESSION | TERMSERVER | USER }

VinsWorldcom@C:\Users\VinsWorldcom> query user /?
Display information about users logged on to the system.

QUERY USER [username | sessionname | sessionid] [/SERVER:servername]

  username            Identifies the username.
  sessionname         Identifies the session named sessionname.
  sessionid           Identifies the session with ID sessionid.
  /SERVER:servername  The server to be queried (default is current).

VinsWorldcom@C:\Users\VinsWorldcom> query user
 USERNAME              SESSIONNAME        ID  STATE   IDLE TIME  LOGON
+ TIME
>VinsWorldcom          console             1  Active      none   5/21/
+2013 7:03 PM
[download]

Not sure if the 'query' command functionality is in Win32::OLE.

Also note that the command 'net user USERNAME' returns the last logon time, but it is reset even if the user locks the screen and then unlocks it.

Notice the last option:

VinsWorldcom@C:\Users\VinsWorldcom> query user /?
Display information about users logged on to the system.

QUERY USER [username | sessionname | sessionid] [/SERVER:servername]

  username            Identifies the username.
  sessionname         Identifies the session named sessionname.
  sessionid           Identifies the session with ID sessionid.
  /SERVER:servername  The server to be queried (default is current).
[download]

I guess I should have been more clear in my description. I'm writing a script to gather
information about remote workstations. I'm using the Win32::OLE module to do queries
against those remote workstations. One of the things I want to get is the name and session
start date/time of the currently-logged in user.

Update: I can get the currently logged on user by looking at the UserName property in
Win32_ComputerSystem, but it doesn't tell me when they logged on.