Here's a use case I'd like to work:
I tell Bitcoin running on my computer or cell phone to run transactions through a bitcoin security service-- maybe I give it a https:// URL for the service.
I tell the security service "auto-approve small-value transactions, but give me a call for any transactions above $X (or $XY per day)."
The security service sends me something in the mail that I keep safe, but that I can use to recover use of my bitcoins in case the security service goes out of business or disappears or I decide to stop paying for the service.
I get bitcoin addresses either from my bitcoin client (not trustworthy!) and/or from the security service that require both my computer and the security service to sign to spend. And I have people send bitcoins (or I self-send my own bitcoins) to those addresses.
Spending coins is done as usual-- I type in an amount and an address. Behind the scenes, magic happens, and if the transaction is greater than $X I get a phone call -- "Press 1 to confirm payment of $X bitcoins to bitcoin address blah, press 2 to cancel."
If I suddenly get random phone calls, I know my computer has been infected.