Few ways:
Click View > Master > Slide Master.
Insert a rectangle autoshape which covers the entire slide.
Double click on the rectangle.
Set the fill to white, transparency to 85%, No line.
Send the autoshape to the back.
Close Master View.
Alternately,
Remove the background.
Click View > Master > Slide Master.
Insert the background by click Insert > Picture.
Send the picture to the back.
Click Slide Show > Custom Animation.
Click Add Effect > Emphasis > Transparency.
Set it to 80-95%.
Set the Start to "With Previous".
Close Master View.
--
Shawn Toh (tohlz)
Microsoft MVP PowerPoint
Site Updated: July 31, 2007
(All amazing PowerPoint skills here <g>)
http://pptheaven.mvps.org
PowerPoint Heaven - The Power to Animate