Hi Boston124,
The post that Jeff mentioned that has more information about the pros and cons of different types of mattress protectors is post #89 here.
Most people are fine in terms of temperature regulation with the thinner protectors that have a semi breathable and waterproof membrane (such as the one that Jeff mentioned) but some people that are more temperature sensitive or are closer to the oven end of the oven to iceberg range can sleep a little warmer when they use them and for them one of the other options may be a better choice in terms of maintaining a more comfortable sleeping temperature.
Phoenix