A lot of people seem to be mirked at the fact that companies have DLC available on Day 1 of launch but don't seem to understand there are good reasons for this. Especially big name games that gather enough attention that people actually notice and complain from the bigger gaming companies.
Lets take Mass Effect 3 - It will have day 1 DLC.
The game itself was finished in terms of development a month ago or perhaps even longer and as a company they will have had potentially hundreds of members of staff working on this game. Once development finished & the game went out to be printed to disks, put in boxes, shipped around the world, delivered to stores and then sat waiting gathering dust until release day. The staff in the meantime haven't been doing nothing - Some may have been moved over to different projects. HOWEVER, depending on time scales and stuff like that big companies often have the same development teams sit there and work on the future DLC and if a story or idea for the DLC has already been fleshed out then putting it into the game isn't that big a deal or hard to do so doesn't take that long especially for a big team with all of the tools ready to drag and drop (exaggeration) models onto a map assign sound and poof its done. That alone is why a lot of games companies reward the patient waiting fans who pre-order with free DLC.
They could quite easily hold it back for a month which in turn would stop a large amount of gamers from sending said copies of the games in to the second hand market where they make ZERO money from.
People saying companies wouldn't get away with this "back then" but 15 years ago gamers were a small percentage of the entertainment market - Now every Tom Dick and Harry has a console of some kind and piracy is rampant (fact. Crysis 2 had 3.92million copies downloaded via Bit Torrent - Side note that is just one of many forms of potential piracy options). As such they have to do stuff like this to get gamers buying the game new and not pirating/using the second hand market.
And before people start saying they make loads of money look at the amount of gaming companies that have collapsed in the past few years and the number of highly anticipated games that have disappeared never to resurface.