A great example of what you're talking about is in Natara's Bonzai
product. You can create a several "task" types: Simple, ToDo's, and
Tasks. Simple items just have a dash in front of them and don't have
dates or completion status. ToDo's have a checkbox in front of them,
have dates (start, due, completed) and a Done/Not Done status. Tasks
have a percent complete bar in front of them, have dates and a percent
complete status. Works really nice and would tie in well with MLO's
Projects and Task ToDo visibility concepts.
You can see a screen shot in their online documentation here:
http://www.natara.com/Bonsai/UserManual/index.html
I would love to see this functionality in MLO....and a Calendar ;-)