Which carriers offer discounts for Phone Only upgrades and when are they available?

If you are looking to just upgrade your phone with your current carrier, you may qualify for a discount when purchasing a new phone.  You will need to re-new your contract for an additional one or two-year term if you accept the discounted upgrade price

 Cingular - 18 months into your current contract for each phone you want to upgrade
 Verizon - 18 months
 T-Mobile