I've never understood that idea that a 30 day exclusivity deal will make people jump ship to the 360. Do they really expect people who have Skyrim for the PS3 or PC, but don't have it for the 360/don't have a 360 to spend 60 dollars for the game plus the cost of a new system just to be able to spend 20 dollars a month earlier? By that time, I'd be strapped for cash, at least in terms of my disposable income, and wouldn't be able to buy the DLC anyway.
Actually, it's a pretty smart idea on microsofts part. Believe it or not but, it was actually cheaper and more cost effective for me to just to jump systems and be done with my PS3 for good. It's a funny story really.
Ok, so I have one the very first PS3 models. It's the 20g version that can play PS2 and PS1 games. But, it lacked any internal wifi unit.
You can probably still remember how bad the PS3 lag was in version 1.0. You couldnt play past lvl 42 or when you reached 10MB of game data. Not to mention that there was this constant Blue/Green screen of death effect in the game after about 2 or 3 hours of play.
So in short, playing skyrim was more of a challenge than an enjoyment. Even the luxury of playing in 1080p couldn't offset the terrible gameplay. Soon enough, I realized that I had to watch how many quests I did and learned to avoid exploring non-quest areas all together in order to keep some playable memory.
So I
had to buy an external wireless adaptor. If I wanted to enjoy playing skyrim again. Which can be very expensive! almost a guaranteed 100$ out of my pocket. Coupled with Dawngaurd results in the overall expense of 120$ of hard-earned cash.
Or I could take the easy way out and jump back to my xbox 360, buy skyrim... again! purchase Dawngaurd, and drop the PS3 for good. Pretty good idea considering that I already have live and the benefit of the market place. Which means it would really just cost me overall 80$ if I were to discontinue paying the monthly fee for live and just let my account degrade back to silver as soon as I had Dawngaurd installed. After all, skyrim is a single player game.

Long story short: Im at a 40$ deficit siding with sony. Yeah, the scale of fate is definitely in the 360's favor here.
But nah, I payed the 100$ and got my PS3 online. I wasn't going to let Sony juice me for a 500$ system that I won't even get to really enjoy.
So yeah, This Dawngaurd DLC definitely has the potential to make people especially with both systems switch sides like that.
Still, this was just my individual case... But, the power to get people to bounce systems is definitely within these 30 day deal(s) Microsoft puts up.