This hint along with this comment are evidence that I'm not the only one who wishes the slideshow navigation panel wasn't always there.
You can adjust the amount of time that the panel is displayed before it dissolves by changing the values in /System -> Library -> PrivateFrameworks -> Slideshow.framework -> Versions -> A -> Resources -> SlideshowConfig.data. You'll need your administrator's password to edit the file. The section you're interested in is down the bottom:
<key>PanelTiming</key>
<dict>
<key>longDelay</key>
<real>5.0</real>
<key>shortDelay</key>
<real>3.0</real>
<key>fadingTime</key>
<real>0.8</real>
</dict>
The longDelay is the delay before the panel starts to fade once it has been used in some way (i.e. if you've moved the mouse over the panel or clicked a button or press a key). The shortDelay is the amount of time it takes to fade if you move the mouse off of the panel.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050904053501814