According to German classifications board Unterhaltungssoftware Selbstkontrolle (that's USK to us normal folk), two trailers for The Lost and Damned have recently been rated by them for the PC, both marked as "gamescom-Trailer". Here's a link to this information.
Gamescom is a new trade fair for the gaming industry, with over 300 exhibitors due to show off their products in Cologne, Germany on 19-23rd August. Rockstar Games are indeed due to exhibit there, so the information appears to check out. No official announcement has come from Rockstar or Microsoft yet though, and of course we can't really trust ratings boards these days, whilst they have been known to accidentally leak information, they've also been known to make mistakes in the past.
Another possibility is that the USK could have easily made the mistake of actually meaning The Ballad of Gay Tony trailers. The fact it says "PC" under the system heading is simply due to the fact the trailers will be shown on a PC. Though one might question the decision to show two trailers at once, and why at gamescom? With all due respect, it's not the biggest audience to show a new product to.
If you search for the original GTA IV trailers for the consoles, you'll notice they're marked as being for "PC", when quite obviously it is meant that they were being released/shown on the medium of PC which if you remember is exactly what happened, it was only later in the day/week that the trailers were put on the Xbox Marketplace and PlayStation Store.
So anyway, take the news with a pinch of salt for now. Basically, we, and in fact nobody has any idea what this is all about. We'll update you if we hear more on this matter. If not then we'll have to wait until gamescom begins to find out what exactly is being shown there.
UPDATE: Rockstar have confirmed that all trailers are released under the PC platform as they are submitted for playback on PC. Which confirms my thoughts on that, and also confirms that it doesn't mean that TLaD will be coming to the PC (though it doesn't rule it out either).