I have seen Noctilucent Clouds as far south as southern West Virginia.  They can be seen even farther south.

Noctilucent: Illuminated at night.

Every spring and early summer Noctilucent clouds are visible in the higher latitudes due to this phenomenon:

NOCTILUCENT CLOUDS OVER EUROPE: The 2011 noctilucent cloud (NLC) season has begun. For the past few nights, observers across northern Europe have spotted velvety, electric-blue tendrils rippling across the sunset sky.

Observing tips: Look west 30 to 60 minutes after sunset when the Sun has dipped 6^o to 16^o below the horizon. If you see luminous blue-white tendrils spreading across the sky, you’ve probably spotted a noctilucent cloud. Although noctilucent clouds appear most often at arctic latitudes, they have been sighted in recent years as far south as Colorado, Utah and Virginia. NLCs are seasonal, appearing most often in late spring and summer. In the northern hemisphere, the best time to look would be between mid-May and the end of August.

NASA is working to understand this phenomenon:

The Aeronomy of Ice in the Mesosphere (AIM) satellite mission is exploring Polar Mesospheric Clouds (PMCs), also called noctilucent clouds, to find out why they form and why they are changing.

The AIM mission has been extended by NASA through the end of FY12. During this time the instruments will monitor noctilucent clouds to better understand their variability and possible connection to climate change. Individual instrument data collection status, as well as spacecraft and instrument health, will be monitored throughout the life of the mission and reported periodically on this website.

The primary goal of the AIM mission is to help scientists understand whether the clouds’ ephemeral nature, and their variation over time, is related to Earth’s changing climate – and to investigate why they form in the first place. By measuring the thermal, chemical and other properties of the environment in which the mysterious clouds form, the AIM mission will provide researchers with a foundation for the study of long-term variations in the mesosphere and its relationship to global climate change. In addition to measuring environmental conditions, the AIM mission will collect data on cloud abundance, how the clouds are distributed, and the size of particles within them.