Changed the code... this works or seems to, on all machines.

Code:

Function GetSIP
   $objSystemInfo = CreateObject("ADSystemInfo")
   $strAuthDistinguishedName = "LDAP://" + $objSystemInfo.UserName
   $UserObj = GetObject($strAuthDistinguishedName)
   $SIP = $UserObj.Get("msRTCSIP-PrimaryUserAddress")

   $msg = MessageBox("Logon Server: " + @LSERVER + @CRLF + 
   		   "LDAP Path: " + $strAuthDistinguishedName + @CRLF + 
   		   "SIP Address: " + $SIP,"Notice",64)
EndFunction