In the documentation for postevent:
itemid The unique number the server assigned to this post. Currently nothing else in the protocol requires the use of this number so it's pretty much useless, but somebody requested it be returned, so it is.
Not exactly true. The ItemID is used when a client wants to retrieve (getevents) or edit existing entries (editevents).
Yeah, maybe I'm being a bit nit-picky, but it is a bit confusing at first. Just thought I'd mention it.