Vodafone online did do 12 AND 18 month contracts. As you say, it's to stop them from having to give away new phones so often I reckon but the lure was that you got more free texts/minutes per month if you went for the longer term one.
Ive never been one to be very bothered about changing my phone every year as long as it's working OK (not being a poseur

) but it's sods law that the first time I go for and 18 month contract I need a new phone 12 months later
Details are in another thread in here on Sony Ericson. It's the K700i and it's by far (by far, by faaaaaaarr) the WORST phone I've ever had in my life. My first one was bigger than a house brick but I'd rather go back to that than have another one of these shitty things.