This edit will remove the text guests see when they hover their mouse over a thread.

The bonus is that your forumdisplay pages will load faster and guests won't be eating up as much bandwidth. Normal members are unaffected.

Open up your threadbit template

Find:
<td class="alt1" id="td_threadtitle_$thread[realthreadid]" title="$thread[preview]">

Replace with:
<td class="alt1" id="td_threadtitle_$thread[realthreadid]" title="<if condition="$show['member']">$thread[preview]</if>">

That's it!

Demo: http://www.utdforum.com/forum/forumdisplay.php?f=2

This is the page size difference: (approx 25KB)
forumdisplay.php for guests = 80.1KB
forumdisplay.php for members = 107.1KB*

*based on a typical setting of 125 in Admin CP -> vBulletin Options -> Forum Display Options (forumdisplay) -> Length of Thread Preview Text

Turning off thread preview for ALL users can be achieved WITHOUT edits through the Admin CP:
Admin CP -> vBulletin Options -> Forum Display Options (forumdisplay) -> Length of Thread Preview Text>Set to 0