There is generally no difference in meaning. Consider the following signs that you may see in England:
"No dogs except assistance dogs" = "No dogs except for assistance dogs" = the meanings are the same, both signs sound fine.
"No stopping except for vehicles making deliveries." This sounds fine. "No stopping except vehicles making deliveries," does not read as well, in my opinion.
Some other sentences:
"I will invite everyone to my party except Andrew." = "I will invite everyone to my party except for Andrew." Both sentences sound fine.
"I like everyone in the group except Tom," = "I like everyone in the group except for Tom."
Edited to add that Woody's example, below re: a scenario is true - "except for," does not work in that sentence. However, I would suggest that you would be far better of using the word "unless," in this sentence than you would "except in the event that..." Generally, it is better to use one word rather than 5, so "Today will be fine unless we run out of ice-cream."
However, Andres is right that, at the start of a sentence, only "except for," works.