To take advantage of this offer, you cannot port your # over. This is for brand new activations only. Carriers always sell their phone for more than you can get it elsewhere. Seems crazy, but that's just how it is. Other companies have deals with cingular to sell their products (but also cell other carrier products) so to be competitive, they are given deals.
I bought my 8700C last week at simplywireless.com for $149 and was able to port my # over. Brand new unit and no rebate hassles.