Most forums, especially ones that use the phpBB software (like The DIS) gives it's users titles related to the number of posts you have made. "Earning my Ears" is just the first title. There are no special privileges or anything when your title changes (once you get past post 10, that's when you can post pictures and links...).
As for when you switch to Mouseketeer, I don't remember. 100, or maybe 500?
You can set your own title by clicks User CP in the toolbar. That will take you to the User Control Panel. Click the "Edit My Details" link on the left. You should see a box called "Custom User Title" where you can set your own title.