In reply to cider nut:
Had our first go at this last night. We'd had a little preparation for it at Aldery cliff the other week, when I seconded a route in the dark. Last night we went to Windgather - somewhere familiar, with easy cracks, just the place to try out night climbing. I led a route in the daylight (well, dusk by the time I finished), then Dan led a route by headtorch. It was quite cold, and we both kept getting numb toes, luckily the rest of us stayed quite warm - we were well prepared at least, I was wearing 6 layers, and Dan was wearing 4 including a thermal and a downy! After the first route we could happily have adjourned to the pub, but I fancied a go too so pushed on and was glad I did as it was quite cool. We both only did VDiffs and Dan's was a one move wonder while mine was overgraded, but you've got to start somewhere!
A couple of things I noticed - bugs! At Aldery, as soon as it got dark woodlice filled the cracks in the rocks and spread out across the faces, so I had to be careful not to crush then while I was climbing. And last night when I was belaying Dan, I noticed earwigs sitting on the rope.
Another thing, but quite an obvious one - if you're placing gear in a corner crack, it's hard to get the head torch at the right angle to check it.
It felt really nice not to be restricted by remaining daylight. Dan and I are quite slow climbers, and normally only fit 1 or 2 routes into an evening session. Last night we did three (and were still in the pub just after 9) - okay, so they were easier routes, but we apart from the cold we could have carried on. It's quite a sense of freedom, and leaves the quandry to when you stop climbing if it's a warm night (I guess last orders would be a good answer to that).