If you are as organized as I am, then you’ll have a million rules set up to organize your incoming email before you even read it. In that case, you’ll be using a smart mailbox to show all the unread email that you have to make it easier for you to read the email. But, the problem we had in 10.4 was that Mail’s dock icon only showed the unread count from your inbox. In the Mail application of 10.5 you actually have a preference to choose what mailbox to get the unread count from. That makes it easier to know what your unread count really is, without needing to rely on mail plugins. Which, by the way, stopped working on me.