Wouldn't know, but an error 400 is usually a bad request. Are you sure you are sending everything over in the correct format that the api requires?
yeah i am sure this same url was working few hours before http://api.search.yahoo.com/WebSear...&adult_ok=1&results=100&output=xml&start=1000
Well first thing I noticed is that you haven't registered an appid and just use the default, perhaps you should register that? It could ofcourse simply be an error on yahoo's part though. (Which should then be solved in the near future) Edit: I don't know the specs but can you go to over 1000 results? It seems if I modify the start parameter to a lower value everything works fine.