SP3 has been "released to manufacturing" i.e. it's been locked down. It does contain a few new functions "which do not significantly change customers’ experience with the operating system". More details in the documents
here.
Microsoft never forced a SP1 or SP2 download, so I have no reason to believe they'll force a SP3 download. What I expect they will do is make it a condition of getting other updates, eventually - like a year later. By then it'll be on magazine cover CDs, if that huge download will be a problem.