1.  Sunlight and heat are the #1 enemy of cakes.  Butter and chocolate melt in the heat of summer.  I will want to setup your cake on a level surface in an area that will stay shaded.
2.  You can have a buttercream covered cake outdoors!  I use a great tasting buttercream that holds up well in warm weather.  I've heard of bakeries only offering  fondant covered cake if the cake is outdoors, not at Custom Cakes NW. Your cake will be delivered chilled, but needs to be placed in a shaded area.

3.  Make sure to display the cake on a stable and level surface that is at least four feet off the ground. 
4.  Cakes are safest when they are displayed against a wall. If possible, consider arranging the cake table up against the wall of a house or side of a tent. 
5.  Wind & pests could also be a factor.  Consider linens blowing onto your cake and what can be done to prevent it, as for the bugs and bees, they like cake too.