How to photograph moving clouds
This is a shot I did last year on Christmas day at night - We always go down to Mish and Fred's (my wife's sister). The clouds were shooting by in the middle of the night; this is a 30 second exposure from the end of the garden.
To get this effect, you need some fluffy clouds moving either directly towards you or directly away from you, not too many (you want to leave some blue in there). Then experiment with various slow exposures - at night allows longer times. During the day, use strong ND density filters or stack polarisers.
Yes, it takes a fair bit of experimentation but it's worth it once you get something nice out. Often, because of the colour temperature and the need to use heavy ND grads, b&w seems to work better as a final print. For example if this had more contrast it'd have more punch.
With a bit more processing it'll make a print that they might like to have just because it's nice to be nice ;)
update Sept 2010 - As per text above, here's a link to another shot of moving clouds during the day. Same technique, different cloud =).