The Jets are going to use their franchise tag on David Harris. Some might fret that this tag, likely worth $9-10 million against the cap will hinder New York's attempts to sign other free agents. It will not necessarily, though. This goes beyond additional cap space the Jets can create.
The two sides are still allowed to work out a long term extension if Harris is franchised. In doing so, the Jets could reduce the 2011 cap hit depending on how the contract is structured. A lot depends on the rules that will be put into place by the new CBA, but this could be a way to create some extra space under the salary cap.
Yes, at the moment that would be Harris' cap hit, but it is important to remember those numbers are not set in stone. Teams frequently reach extensions with franchise players. I would imagine the New York Jets will attempt to do just that with David Harris. The tag as much as anything will provide a safety net just in case that does not happen.