Over on the official forums, Mike Ferguson has shared some of the lesser known details about World Vs World. First off he explains the despawning aspect of siege equipment. Unbuilt siege currently has a timer of 10 minutes before it is automatically destroyed, while completed siege has that limit upped to 30 minutes. That timer is reset every time a player interacts with it, so it's important to keep a good portion of your siege refreshed throughout your matches.
Secondly, we have the siege limit. In a single 1,000 unit radius players are able to throw down five siege weapons. Any attempts to place more than that will be met with an error and force that siege equipment back in your hands to be placed elsewhere. We also have a total siege limit that covers the entire map. Mike explains that the details are tough to explain in a simplified form, but notes that there is a separate limit for Flame Rams and a separate limit for any other type of siege equipment.
The topic of the player limit in WvW as well as various stats such as class percentage and k/d ratio also came up, but exact details and numbers were unavailable. However, there is mention of their desire to allow players to trade siege equipment rather than resort to the less traditional method of drop-trading, but coding issues prevent that at this point in time.