If you plan to go in 2009, the Annual Pass Premier is good value for Universal and Islands. If you order before the end of the month you get a two year pass for the price of an one year one.
I worked it out to buy an Annual Pass plus a Sea World and Aquatica 14 day ticket is just £10 more expensive than buying a Flexticket 4 park with the advantage that in 2009 we won't have to pay anything to go in Universal/Islands.
Usually I buy when I think the price is right, usually before a price increase which I think is usually in January. Though you have to make sure the Disney tickets are for the year you are going to use them.