Early studies have suggested that noctilucent clouds were caused by space shuttle launches. The space shuttle exhaust plume is composed mostly of water vapor. As the shuttle launches into orbit, it can dump significant amounts of water vapor into the upper atmosphere as it passes through it. More recent studies by Dr. Michael Kelley have added credence to that idea: shuttle launches in 2003 and 2007 produced corresponding noctilucent clouds.
NASA launched a satellite called AIM in 2007 to study noctilucent clouds. AIM stands for Aeronomy of Ice in the Mesosphere. AIM continues to study the phenomenon using several instruments, including two imagers and a meteoric dust measuring device.
The part I find ironic is that NASA launched a satellite to study a phenomenon caused... by the launch of NASA satellites!
Update 2009-07-30: Added noctilucent cloud image from Wikipedia.