This is how to cancel a Wells Fargo check as of Sept 21, 2011.
Login to your WF account.
Click “Account Services”, it’s among the top tabs the 5th one counting from the left.
On the next page, Under “Account Services”, find “Stop Payment on a Check”, should be the 9th link.
Then just follow the steps.
You will need to have the info on the check #, the amount, payable to, and signed date.
Update: It turns out there is a STOPPING PAYMENT FEE of $29… =(