Will the Nexus 7 have a significant impact on the Android Tablet ecosystem?
Plenty of people don't ever order online if they've never held the thing in their hands and played with it.
There's a big bonus to walking into a Best Buy, playing with a device, then walking out with it.
Let's hope Google doesn't mess this up.
Answering your question though, as long as the numbers add up, devs will have to support it.
What I mean by this is that I would look at the most popular tablets, and make sure my App works fantastic on them so I cover the majority of the public. I think developers like me will focus more on making their app look pretty on the 7 inch form factor, and possibly move 10 inch compatibility to the backburner. Will it get more tablet apps to market? I don't think so. If developers wanted a tablet compatible app they would do it regardless of what is on the market.
There is nothing "difficult" in the SDK with regards to tablets, it is really the same as developing for a phone. Just the screen is bigger, and you need to take extra care to make things look pretty. However most of the effort goes into the phone side of development because the apps will be installed on phones more than tablets in terms of sheer numbers