Can a GSM phone work in a CDMA network?

The answer is "Yes, but carriers don't like it." There is no technical reason why you would not be able to use a CDMA phone on any CDMA network, but in order to ensure that they make money, carriers will only allow ESNs (serial numbers) from their own phones to be activated. The exception is (or rather, will be) Verizon Wireless when they institute their Open Access Policy. This will allow you to use any CDMA phone on their network.