Since we all know that PS will be released Q1/Q2 of this year, and there hasn't been much known about it (not even a trailer) that means they have to start getting press releases and marketing the game if they want to release this game with success.
GDC 2011 starts late February and goes on until early March. Among the exhibitors are
Sony Computer Entertainment and The9 (Planetside publisher).
GDC is a huge event for new releases and if the Planetside developers want this game marketed and want PC gamers to know about it, this convention is the best place to do so.
E3 is great but way too late (June), and ideally Planetside should have trailers, videos, pics and even the beta before then, that is if they are confident they can release it no later than Q2.
That's my theory. I hope PS gets the recognition it needs to become a successful MMOFPS.