This is another thread from a while back about this topic -
http://koolie.net/exons/index.php?showtopic=1394There might be other times before I was a member that this was talked about too. It's such a common problem for puppy owners.
The 2 main points I'd make about it are that (while I'm not suggesting that you ought not try to manage it in the short term) you shouldn't worry that it 'means anything' about the future behaviour of your dog. Lots of ppl worry that it relates to aggressive behaviour when the dog is older, but there is simply no evidence that this is the case. This causes them sometimes to use fairly heavyhanded strategies to prevent it, and the ironic thing is that having used these strategies are much more likely to result in aggressive behaviour in the adult dog (who will have less trust in ppl).
The other is 'be prepared'. Expect that this will happen and be ready to respond to it. Try to divert the pup before he starts - are you teaching him to tug with toys? Effectively that is the same game, and you should be able to get him doing that before he starts on your legs. When you play that game with a pup, try to make the toy act as much like your leg and trousers as possible - move it, pull against him, at times also provide a passive force that he can pull against, 'reward' him by letting the toy move towards him as he pulls on it and by letting him pull it out of our hands. Learning to play tug (for the ppl) well is a skill - take Brick's feedback on whether the game is working or not.
Susan Garrett is one of the most amazing trainers and her dogs are really good tuggers - she wrote this about building your dog's interest in playing with a toy -
http://www.clickerdogs.com/createamotivatingtoy.htmPutting a bell on his collar will reduce the chances of him catching you unawares.
I can see where you're coming from with trying to ignore the behaviour, and I think your idea is a good one. The thing to keep in mind though is that the general advice to ignore certain behaviours that you do not want your dog to do is based on the belief that doing so will mean that the dog is not rewarded for doing the behaviour. For some behaviours that is the case, but with this one, while you interacting with your pup as a result may be a reward, it is not the only reward - he can have a good time biting you and pulling your clothes even if you keep still and don't interact with him, and that will certainly reward this behaviour (at least in the short-term - even if you don't manage to stop him doing it now, chances are very high that he will grow out of it anyway).
For times when all else fails, you can 'punish' him effectively by taking yourself away. This only needs to be very short - 10-20 seconds. I would be worried if you were using timeouts much though.