Threaded comments are surprisingly difficult to model in a relational database. I spent about a month on the problem when I rolled my own threaded comment system for newsley. Relational Databases really don't lend themselves to easily modeling tree structures.
He has two really good chapters that specifically building a comment system. It takes a bit of work, and you end up pre-computing the comment system on INSERT's instead of SELECT's. But, you can build a comment system that pulls your entire comment tree in order using a single SELECT ... FROM COMMENTS ... ORDER BY comment.node_left;
If you're interseted in doing something like this, I'd highly recommend this book: http://www.amazon.com/Hierarchies-Smarties-Kaufmann-Manageme...
He has two really good chapters that specifically building a comment system. It takes a bit of work, and you end up pre-computing the comment system on INSERT's instead of SELECT's. But, you can build a comment system that pulls your entire comment tree in order using a single SELECT ... FROM COMMENTS ... ORDER BY comment.node_left;