Here is what I used for my daughters wedding cake last week... can't be much hotter than what we just went thorugh. 106 degrees with high humidity. The day of the wedding was 95 and very humid...
I used real butter (always do)
1 cup butter (room temp not too soft)
1 cup cream cheese (I used the soft philly)
mix the above together very well and start adding powdered sugar until you get the right consistancy you need for icing. If you get it too firm, add a little milk until it is good.
The icing did not leave the sides of the cake at all... it was in an ac room until 4 o'clock then I set it up and it set there until 8 p.m. I was a bit nervous that it would be a mess but when they cut it the icing under the fondant was as good as it was when I put it on. I was thrilled.
