You can do that with highslide, for example.
http://highslide.com/
I integrated it into a database frontend I did with vB-technology:
http://www.roma-antiqua.de/forum/hotels.php
It's German, but you can see it in action if you click on the small blue "i"-icon (for information) on the right or on any of the symbols in the second to fourth column of the table. You could show a whole post or thread in there alright, I guess - via Iframe, in this case, too. One would have to replace the links inside forumdisplay template and probably remove navbar from showthread template.
It might have a real good use for the showpost function. Wouldn't that be handy if the link to a single post would just let the post slide open like that?