Additional notes : no, it's not an earth-shattering extension, since the default behavior for the preview button is pretty fast, but this mod is intended for those of us who like to live in the warp-speed lane
(also, because I use the "preview" button about a zillion times before posting)
If you really want to mimic the "preview" button's original behavior, you should use this on step 13 of the install :
<!-- MOD AJAX post preview -->
<p><input type="submit" name="submit" value="<?php echo $lang_common['Submit'] ?>" tabindex="<?php echo $cur_index++ ?>" accesskey="s" /><input type="submit" onclick="xajax_getpreview(xajax.getFormValues('post')); document.location.href='#punwrap'; return false;" name="preview" value="<?php echo $lang_post['Preview'] ?>" tabindex="<?php echo $cur_index++ ?>" accesskey="p" /><a href="javascript:history.go(-1)"><?php echo $lang_common['Go back'] ?></a></p>
<!--// MOD AJAX post preview -->
This tells the browser to go to #punwrap, the top of the page, whereas my version goes to #ajaxpostpreview, the preview block.
It doesn't work so well with Opera, because the document.location.href = '#ajaxpostpreview' instruction seems to work only once... if you click "preview" again, the window doesn't scroll back to the right place (Opera's lazy, man). If anybody out there has a fix for this, I'll be glad to know it.