Hey, I don't know if you have been looking out for these, but they make very nice subjects.
More on what they are here:
http://en.wikipedia.org/wiki/Noctilucent_cloudI was out around midnight (in northern Germany on the coast) and noticed the first clouds I have seen so far this year. The texture of these clouds is really wonderful and worth looking out for if you are out in the right latitudes and weather.
Not the finest shots ever, but I think you can get a feeling for the subject. Maybe I'll have some better shots in the course of the Summer.
Cheers,
Markus
P.S.: Panasonic GF1 20mm - 13sec @ f5.6 and iso 100 processed in LR3...