If you look inside the thread, there's a date besides each post that marks the time when the post itself was posted. So, the date for the first post would be when the thread started.
Basically, I guess it's possible to add a hack that would display the date when the thread started at the forum page - but I think having both dates might be too much.
I know it is inside the thread, I was just wondering if it could be outside so you can tell when it was posted without haveing to take the time to open it.