I dont think anyone has a 100% accurate answer. I heard a full section, which makes sense since if you add 30 that gets divided among sections and seating in classes and stuff might become an issue. The way HBS runs their classes its actually easier to add an entire section in a lot of ways.
I understand why you asked the question though but really take a breath and dont over think things like this since it is completely out of your control. You guys will see the numbers this fall since the increases were supposed to be for the class of 2011. In the end it really doesnt impact applicants all that much other than making it about slightly easier to get in as was pointed out. Yes it will trickle down to other schools so in the end it will make it easier to get into every school if there are more slots at HBS, Stanford, and Wharton. If it is 150 or so, that means 150 more people going to top 5, top 10, etc than before. I dont think anyone would turn down HBS because of this decision alone.
There are a lot of other factors that could have potentially significant impacts on the number of applications. Think of what changes to student loans, career opportunities post MBA, overall economy, etc.
_________________
Kellogg Class of 2010...still active and willing to help. However, I do not do profile reviews, don't offer predictions on chances and am far to busy to review essays, so save the energy of writing me a PM seeking help for these. If I don't respond to a PM that is not one of the previously mentioned trash can destined messages, please don't take it personally I get so many messages I have a hard to responding to most. The more interesting, compelling, or humorous you message the more likely I am to respond.