I have been logging outside temperatures with an Arduino Mega, the DHT22 and the Virtuabotix DHT22 Library for several weeks. When the temperature drops 0 C (32 F), I get checksum errors. When the temperature goes positive (0 C or above), it works fine.
I used the example program to verify this error, and I got the same results. I tried the Adafruit library and it works fine without error. No checksum errors with the same hardware, just different library.
Some of the components within the microcontroller and/or the DHT22 may not be rated for that low a temperature. I use a thermistor on a long cable and have my Arduino UNO board and LCD inside where it is nice and warm. Make sure your Thermistor is secured to the cable with a water proof connection to prevent shorting. Outdoor Silicon gel caulking applied to the connection of thermistor and cable should do it, but test with an ohm meter by dipping it in water to see if there is a short.
Thank you for letting me know about the problem. I will re-post on this question once I have had the time to investigate the checksum algorithms and get back to you with an updated version of the Virtuabotix DHT22 Library that fixes the problem, or an identification of what else may have caused the issue.