Ah. Small problem that needs to be looked at.
1) If, for example, the first 500 characters of the opening post include something that you as an admin edit out - they will continue to appear in the preview.
2) If, for example, you split a thread then
---> a) the newly created thread will not have a preview; and
---> b) if you, for example, split the thread so as to remove the first post (say, to your trashcan forum), the that post will continue to exist in the 'preview'.
What needs to be done, is adding an additional hack code to the editpost script and the splitting action script. In both cases, when a thread is updated, the 'preview' must be updated as well.
I'm off to sleep now. I'll see if I can help you when I wake up, if that will be needed