Once you purchase the iPhone 4 (discounted through the carrier with a 3 year contract), the phone is yours and you can do what you like with it, including selling it when the next iPhone comes out.
I would think that an iPhone 4 32GB would have a closer resale value this spring/summer to $500. As an example, last summer when the iPhone 4 was close to being released an iPhone 3GS 32GB could be sold for $500ish if it was in very good condition.
I would think that an iPhone 4 32GB would have a closer resale value this spring/summer to $500. As an example, last summer when the iPhone 4 was close to being released an iPhone 3GS 32GB could be sold for $500ish if it was in very good condition.