moved from Kubuntu 8.04 to 8.10 and KDE 4.

Under KDE 3, in Konquerer, when I paused the cursor on a file, a small window would pop up with a preview of the file (text, image, etc).

Has that disappeared in 4 or is there some way to turn it back on?

In in the configuration dialog, in File Management > Views, there's a checkbox called "Show tooltips". That should do it.