IMO, I think it is too early for Rockstar to be releasing their first trailer.
Actually, it seems too early for them to have told us anything but the name but here we are with the information that there will be custom content over Xbox Live and what physics engine will be used.

I say its too early because Rockstar has never been a company that releases too much info before the release. As I recall, it was about 4 or 5 months from the release of San Andreas before I think even the name was released.
I actually wasn't expecting anything from Rockstar until atleast May.

But apparently the first trailer is on its way and I would like to share my thoughts about what I think will be in this trailer:

I think that this trailer will be similar to the L.A. Noire trailer. But of course, Rockstar could easily just be "sneaky sneaky" by making this trailer similar to the Manhunt 2 trailer since all that trailer shows is a few flashes of the characters and setting. But if Rockstar did that, I feel people will be pissed since judging by the people on the GTAIV forums, they have high hopes for this trailer.

But I think this trailer will just contain the bare basics, setting and main characters put into a short will that I think won't last more than 60 seconds.

I think that is the reality since many people want this trailer to contain actual gameplay. When I see people who are hoping for that, it makes me laugh inside because even though Rockstar has released a lot of information very fast for the first time, I doubt they will show gameplay.

But after the trailer is released, I think Rockstar will release artwork that contains cartoon like drawings of the main characters with scenery from where this game will take place.