Liefeld is pretty gracious in glossing over the apparent fact that Marvel took his and Valentino's pitch for Young Avengers and created the same team under a different name without crediting them. But I guess that's the work-for-hire life.
Edit: Apparently at his (now-defunct) blog, Tom Brevoort claimed that work on New Warriors was already underway and it just happened to have almost the same cast as Liefeld/Valentino's "Young Avengers." I'm not sure that sounds plausible to me, but maybe Marvel editorial already had given out some ideas about which characters it wanted to use.