I got interested in this topic and compared prices for three phones in my country: the iPhone 3Gs, the Nokia N97 and the Nokia N900. I took the original price (day they became available here) and the price 6 months later. Prices are taken from an online service that lists cheapest available offers.

Here are the results expressed as "price after 6 months as percent of original price":

iPhone 3Gs: 79% of the original price
Nokia N97: 82% of the original price
Nokia N900: 87% of the original price

In other words, somebody who bought an iPhone 3GS on day 1 lost 21% of his money if he sold the device half a year later. N97 owners lost 18% and N900 owners would lose only 13%.