That is a whole big copyright discussion…but here is what it says from copyright.gov
“How much do I have to change in order to claim copyright in someone else’s work?
Only the owner of copyright in a work has the right to prepare, or to authorize someone else to create, a new version of that work. Accordingly, you cannot claim copyright to another’s work, no matter how much you change it, unless you have the owner’s consent. See Circular 14, Copyright Registration for Derivative Works.”
There are other things involved, like how long the work has been copyrighted, etc. For example, after a certain number of years, a copyright works becomes pulbic domain and can then be used for derivitive works.
The whole issue is a little more complicated with patterns since there is a somewhat limited way to design certain things. Here is another discussion more relavant to knitting patterns at knitty.com: http://knitty.com/ISSUEfall03/FEATcopyright.html