SPOnG have been in touch with Rockstar and say they were promised that the trailer will "not be one of those annoyingly short teasers", but instead will be the first substantial look into what GTA IV is all about.
Interestingly though, they also claimed they will be recieving a bunch of new screenshots from the game which will be shown on the SPOnG site along with the high definition GTAIV trailer. This is the first we have heard about screenshots being given out to sites, as to this point we were only expecting a trailer. Linkage.
UPDATE: SPOnG have now changed their story, presumably at Rockstar's request. It looks like we won't be seeing screenshots afterall...