Oh dear, I seem to have given the wrong impression. We have been thinking about - and researching whippets - for a while, so although I only posted on Sunday, it wasn't like we'd just thought of it, although I didn't realise we would get one that week!
Anyway, at the risk of sounding a bit defensive, here's some more details. The dog - Freddy - is 1 years old and is socialised around other dogs. Although he's been in kennels, he's been showed and well handled, so no dodgy history/abuse issues (which was a concern if we'd got a rescue greyhound - we have a five year old). We met his mother (who is a family pet as well as show dog) and met the kennel man and owner. The owner said we could wait and get a puppy from him - due in a few weeks - or we could try things out with Freddy, as he was a very bright dog and lovely natured. If it didn't work out, he'd take the dog back.
The first day was a bit tricky - the jumping up thing especially - but he seems to have already got the message that this is wrong and is doing it a lot less. We've just been consistently saying no and pushing him gently down. Seems to be working. And last night he fell asleep at my feet for a couple of hours, so seems much more relaxed and contented, even after just two days. (First night he paced around quite a lot).
I took him with my son to school, and he wasn't too keen on all the buggies going by, but didn't freak or anything (just seemed a bit nervous. Obviously hasn't come across them before).
Otherwise, he is acting pretty relaxed and happy; very friendly, no barking, happily going in his bed for a rest. Chewing at things a bit, but I'm assuming this is normal puppy behaviour.