Since many people on HN will eventually find themselves having to care about the distinction between an "employee" and an "independent contractor", because hiring the first costs you extra taxes: if a person isn't allowed to outsource themselves, that is strong-but-not-dispositive evidence that they are an employee. If you sign a contract with a consultant, it is highly likely that the consultant has the right to direct fulfillment by anyone he thinks capable of giving you what was agreed upon. He has wide discretion in picking subcontractors, just like he also has wide discretion in picking what hours he works, how he performs the work for you, what tools he uses to perform the work, what other employers he works for, how he comports his professional affairs, etc etc.
I've seen a boilerplate contract or two for consultants which forbid them to do this. If your boilerplate has that, you might want to ask your competent legal representative on whether there is a risk of a contractor being retroactively reclassified as an employee by the IRS. (You profoundly don't want that. Grellas has a magnum opus on it here: http://news.ycombinator.com/item?id=1137930 )
Similarly, as a consultant, it is probably to your advantage to assist your clients in meeting their legal obligations by e.g. preserving their lack of apparent or actual control over you.