Like many users of MT, I use javascript to call a pop-up comment window. What I didn't realize though is that 11% of internet users don't use Javascript and it can cause problems for people who are disabled because their browsers don't support it. Mark has posted instructions on how to replace the code so that you can still have pop-ups but also have a solution that works for other browsers. You can find it here. He has pointed out that the latest default templates in MT now "get this right" – but if you're like me, I don't normally upgrade my templates every time I upgrade MT.
I should also point out that Mark is doing a whole series on 30 Days to a More Accessible Weblog and there are a lot of other really good tips worth reading there!