While this question is similar to others about securing wallets, I think it merits a question of it's own.
One thing that has dissuaded me in the past from running an online wallet service of some kind is the worry that I could be compromised (rubber hose, extortion, intimidation etc) and by extension permit full access to a large collection of bitcoins that I don't own. If I'm running an operation with potentially millions of dollars worth of bitcoins and my contact details become known (not that hard to find really) then I'm naturally a risk to my clients.
I've heard of multi-key encryption so that no one person has complete access (client PIN etc), but I'm not sure if that would solve the problem.
So my question becomes how can a single person operation ensure that they are unable to compromise the wallets under their care? Ideally the answer would not require any technical knowledge on behalf of the client beyond a passphrase.
(If such an approach became commonplace then it could act as a deterrent because attackers would simply see that the website uses that approach and slink away.)