This is not a real well thought through solution. Basically a key is missing. Using this method assumes that the ubuntu key server has the right pointer. But it is a fast solution.
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2EA8F35793D8809A
Here is the command to add the missing key mentioned in the error message.
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 2EA8F35793D8809A
So I take the missing key
2EA8F35793D8809A and ask the keyserver.ubuntu.com to add it to the list of keys I have in apt.