It's funny that you mention it as I wrote something that might be what you are looking for just a few weeks ago.
(edit as I re-read: doubtful that this fulfill your needs, you are probably better off with a more traditional Bugzilla-type-of thing. Keeping this posted anyway :)
This is an application that I wrote for internal usage. Have you ever heard of TeuxDeux (http://teuxdeux.com/)? Well, basically, I fell in love with the user-interface and it fulfilled that part of my needs. So, I took the look & feel and ran with it and made the following additions to my own version:
- Under the "some day" items I added categories which could be edited by dragging & dropping the category into the "some day" input box.
- Encouraged the usage of "hash tags" to TODO-items (#projectabbr Fix nasty crash bug)
- Added a filtering box which would highlight queried TODO's with that search filter
- Made it so that when you hover over a todo-item during drag/drop: a tree would pop-up with issues that depend on parent. You can drop the todo-item you are dragging and dropping into any of the items in that tree. When a parent is "freed" (done), its immediate children become separate TODO's in the list again. Rince/repeat.
- A detail screen which would appear when clicking on "..." next to a TODO item.
Put in mind here that I like the minimalistic and straight-to-the-point of this layout. I want my TODO lists to act like post-its :)
Unfortunately I have not open sourced it and I really never intended to do that for the very reason that I basically "ripped off" the look & feel of Teux Deux. But this might give you a few ideas, though. And it's not really too hard to implement. Give me a holler if you need a bit more info :)