In the July 18 patch for Team Fortress 2, we got a handful of the usual bug fixes, and some unusual Grockets popping up in the distance. We have no idea what these are, or what they mean. Valve has a history of doing crazy things like this to get the community excited for upcoming announcements.
The Steam forums are filled with players' guesses as to what these Grockets might mean. Of course, every time Valve does anything cryptic, we have excited speculation that this means Half-Life 2: Episode 3 is finally coming out.
I'm sure that it won't be anything that exciting. Maybe Valve is just doing this to mess with our heads.