I would release it for the following reasons:
1. It seems to me that the value in software of this nature, lies in the code, not in the graphics. Otherwise, the apps would not be skinnable! You aren't copying his app.
2. you didn't take his work and copy it... You made your own version of it, for a different piece of software altogether.
3. You aren't selling it, nor competing with centrafuse in a non-profit fashion.
Then again, if you have any qualms about it, just ask david...