I was just pitching a client that was on IE 8. D'oh! Very hard to keep things working on older stuff. That said, it seems very surprising that overall usage of IE was so low. I would have thought it to be at least 20%.
So the truth is that Trello always stayed pretty much ahead of the IE usage curves, so that most people with IE willing to use it, simply couldn't.
I guess this is the reason why they don't have many IE users in the first place.
[1] according to W3Counters, which is the first I found with 2011 numbers readily available.
But my point that you're misreading numbers remains. You see low IE9 usage because you don't have IE users, because, in the previous years, up to 80% of them couldn't use your product.
And now you're using these numbers to back your decision that you don't need IE9. Guess what? You won't see IE adoption grow much over the next years as well.
IE users means enterprises. By dropping IE9 now, you're again running faster than them. That's perfectly fine and I'm actually jealous, I wish I could do the same. At the same time, I'm sure it has a measurable impact on the potential market of your Trello business class product.