Support, downtime, and "unforeseen events" are what I've been struggling with lately (in terms of thinking up a business plan).
The sorts of things you're talking about - because as far as the client's concerned, if their site isn't up, it's your fault - even if your host did something naughty or a hurricane wiped out the datacenter.
Perhaps a clause in the contract stating that I will not be held liable for any technical issues beyond my control. Etc, etc.
Something to think about.