I'm looking for examples of actual websites (not articles/demos) that use a modal dialog (like a jQuery dialog) to display comment or contact forms, in order to get some ideas of how I might want to implement similar functionality.
I'm having some difficulty finding such sites via search, though.
Well, I could definitely point you to an example of a mooTools moodal box. http://www.e-magine.ro/playground/moodalbox/
Is your question more of how the form layout should flow within a modal box? I would keep it short and sweet and try to avoid ridiculously long scrolls.
You may want to consider using a library such as moo for it's transition effects as they are much smoother then jQuery.