I use one of
these dealios with a cart skirt (black, fastens to the cart with velcro). Laptop sits on the middle shelf (hidden) and I have a Viewsonic 19" flat panel (widescreen display) which was only $249 at CompUSA. The cart was a little over $100 and the skirt (which I had to buy from a commercial AV equipt dealer) was about $50.
I use a Sandisk card reader and only upload the small jpegs. (Takes maybe 3 minutes for ~1200 images)
I do event cards and I fan them out all over the top of the cart (so no one is tempted to set a drink down on the cart).
I prefer to put the show near the bar or the gift table. I figure if there is anything shady going on...a crook would probably steal the couples wedding gifts (esp. the envelopes) before they'll mess around with something that looks like it's fairly secured. (Most people -- including most of my assistants! -- think that the reception site provides the AV cart so I guess that adds to component of "there are too many eyes around for me to mess with this thing."
One of the nice things about the Viewsonic is that it's BRIGHT...I can pretty much see it from almost anywhere in the reception hall ('cept the potty).
The downside to slideshows is that when you are ready to pack up and leave...that's usually the time you have 20 people gathered around your slideshow!!!