My answer: not a lot, actually. You are investing in a professional performance and you should therefore never have to worry.

A few examples:

Traffic congestions!
There will be traffic jams everywhere and all the time, so that is no excuse (nor are flight delays). You may expect me to always calculate for the unexpected. Especially when I am the last speaker, I will try and listen in to most presentations before me, so I will be largely on time.
Should the interval between two performance be rather short, I will always discuss the (im)possibilities with you.

Guido is ill
I am hardly ever ill and when I am, you can still find me on stage. Rough remedies haven’t been invented for nothing. So: high fever, sprained ankle, major back pain: it has all happend to me in the past 20 years. The audience hardly noticed.

It is really impossible for Guido to be there
Arrested for passing on secret documents to Wikileaks? Found unconscious under a bar stool? Abducted by aliens? Eaten the wrong mushrooms? Enlisted for military service in Afghanistan? It has never happened, but one never knows.
And should it happen: all top speakers are good buddies so we will provide a perfect alternative!

We have made drastic changes to the programme
Great! Improvising is what I live for! I got a phone call once to check whether I would be able to perform forty-five minutes later at a conference because a speaker had missed his flight. Travel time: 30 minutes. I was there and my scores equaled those of carefully planned presentations. So challenge me and make nmy day!