This is one reason why Mint would never work in the UK. We have real security on our banking websites - hardware security (You get sent a hardware device to generate codes, or to put your card into etc).
(As an aside, our banks are simply crazy fast now. I logged into HSBC the other day, issued a bank transfer to a Barclays account. Logged out, logged into Barclays less than a minute later, and the money was there. Impressed I was).
Any banking website that simply accepts a username and password would scare the hell out of me.
I still don't quite understand what the value-add of mint is though. I'm guessing that banking websites in the US aren't very useful, or that some users value being able to collect all their financial data into one place (How many banks accounts from different banks do people need?)