I'm curious to hear what other people think. I shut down ClearCoin when the value of its wallet threatened to get so large I was no longer comfortable self-insuring it with my own personal bitcoins. It is still shut down because I haven't found a group that I would trust to run it securely (and I don't want to take the time to raise the money and hire experts to run it properly).
We are starting to see well-capitalized, experienced companies creating bitcoin businesses; at this point, I think if you are holding other people's bitcoins then having insurance or posting a bond to self-insure up-to-some-value for each account is reasonable.
In my view, if your business model for your free service doesn't cover the cost of insurance or a bond, then you're just pushing a lot of hidden risk onto your users.