It must check if EventID of the last log entry matches parameter.
in ARGUMENT need to accept single number of EventID.
trigger for example:

{Host:eventlog[System].logeventid(1001)}

=1

Use this feature very useful for Templates if there are servers or workstations with different Locales, as the text of messages in them are different but number of EvenID is identical.

I think this feature was not just because the support of EventID generally appeared recently in v1.8.

Something of this in the first post: http://www.zabbix.com/forum/showthread.php?t=14944