What I look at to determine if a test should be automated is:
How difficult is it to automate?
If I have a test that takes me 16 hours to automate and 2 minutes to execute manually. I would have to execute the test 480 times (On different itterations of the application) to break even on the Manhours spent on the test. (Not a good canidate)
How many times will I have to execute the test?
Is this a test we will execute a 6 dozen times and then expect to retire it? Or will this test become a key piece in determining if the system is ready to go out the door.