Chris,
Derek has, as usual, uttered the wise words of a seasoned professional !
Perhaps the safest disclaimer is to simply say " In my considered oppinion the posibility of worsening the situation by proceeding are such that it would be unprofessional of me to do so."
This said, there have been many occasions where I have proceeded beyond the point of wisdom.
On these occasions it has always been based on not only I disagreeement of the technicalities of the task, but my judgement and knowlege of the client, and with the client fully informed of the risks involved.
So far this has not backfired on me, but on some of these occasions I could not honestly say how much luck entered the equasion. From a commercial point of view, many of these decisions to proceed were wrong.
As Derek has pointed out, many disclaimers issued would, if tested in court, be declared 'unfair conditions of contract'.
By not proceeding you may be branded "The man who couldn't get the stain out", which is infinately better than "The man who ruined my carpet - but I made him pay for it"
On the other hand if the customer understands the reasons you refused, in doing so you can earn the customers' respect.
John.