This might help anyone who has an iP4500 printer which is no longer serviceable, out of warranty but still less than two years old.
Speaking to a Canon Service Centre, I learned that as a goodwill gesture, Canon allows them to take your old printer plus £33.35 in exchange for a brand new iP4600.
I know it works - because I did it today! I paid just £33.35, handed in my old 4500 and walked out with a brand new 4600.
I don't know whether you can trade in any other Canon printer. Might be worth a call to your nearest Service Centre (details of them are here: http://www.canon.uk).
Hope this helps someone.